Analysis: Vice-presidential selections have had little electoral impact in recent elections. John Edwards didn't help John Kerry win the 2004 election, or even Edwards' home state of North Carolina. Paul Ryan didn't win Wisconsin for Mitt Romney in 2012.
In 2008, Barack Obama was a huge favorite over John McCain before either man picked his running mate, and Joe Biden and Sarah Palin made the race more interesting but didn't change the outcome.
That said, the selection of a vice president is so closely watched because it provides clues to the candidate's view of herself and the electorate, as well as insight on the strength of different coalitions in each party who want the vice-presidential nominee to align closely with their interests. The vice-presidential selection also often creates a new national leader in each party, because the role comes with so much attention. Sarah Palin and Paul Ryan are now very influential figures in the GOP, and Biden played a key role in governing the country the past eight years.
So with Donald Trump nearly assured of being the GOP nominee and Hillary Clinton closing in on capturing the Democratic nomination, here's a look at some of their potential vice-presidential picks:
Biggest Potential Boost to Candidate's Image: South Carolina Gov. Nikki Haley, New Mexico Gov. Susana Martinez.
Clearest Path to 270 Electoral Votes: Ohio Gov. John Kasich, Florida Sen. Marco Rubio, Florida Gov. Rick Scott, Wisconsin Gov. Scott Walker.
Biggest Potential Benefit to Governing: South Carolina Sen. Lindsey Graham, Rubio.
Wildcards: New Jersey Gov. Chris Christie, Texas Sen. Ted Cruz, Iowa Sen. Joni Ernst, Oklahoma Gov. Mary Fallin, former House Speaker Newt Gingrich, Maryland Gov. Larry Hogan.
Trump is struggling in the polls with female and minority voters in ways that could doom him in a general election. Getting the Indian-American Haley or the Mexican-American Martinez to join his ticket would be a huge coup for the real estate mogul. It would allow Trump to brag to voters that a high-profile minority woman opted to run with him, buttressing Trump's argument that he is not racist or sexist, as some of his critics argue. Haley is perhaps most known for supporting the effort to get the Confederate flag taken down from the South Carolina capitol grounds.
Haley and Martinez have both distanced themselves from Trump during the 2016 campaign, with the South Carolina governor delivering a response to President Obama's State of the Union address in January that was as much anti-Trump as anti-Obama. If either woman joined his team, Trump and his team could argue that the candidate has evolved and grown so much that one-time critics now fully embrace him.
The goal in such a selection would not necessarily be to get Asian-American or Latino voters to back Trump in large numbers. Having a minority woman on the ticket would also be a signal to white voters and women who agree with Trump's policy stands but don't want to vote for a candidate who is perceived as intolerant.
Would Haley or Martinez take the job? Right now, there is a very strong anti-Trump movement in the Republican Party. But if Trump is the party's nominee, someone must serve as his running mate. Haley and Martinez project the inclusive image that GOP elites wanted at the top of the ticket.
And for the women themselves, it may hard to turn down the job of potential vice president if it is offered.
Ernst, who the Washington Post's Chris Cillizza floated as potential Trump running mate, is not a person of color, but her service in the Iowa National Guard (including a deployment to Kuwait from 2003-2004 during the Iraq War) would be a unique credential. The downside would be that a ticket with Trump and Ernst would have little government experience. Ernst has only been in the Senate since 2015, and Trump has never held elective office.
In contrast, Haley and Martinez are in their second terms as governors. Fallin does not have the national profile of these other three women or a particularly compelling biography. But she is also a second-term female Republican governor.
Haley and Martinez are not from key electoral states. Picking a running mate from a swing state is a regular tactic for presidential candidates, even if it often does not work. And the big problem with assuming Ernst, Kasich, Rubio, Scott or Walker could carry their home states in November is that they all won elections in 2010 and 2014, when voter turnout was lower and more conservative-leaning. None of them won when the so-called Obama coalition was out in full force, as is likely this November.
If he selected Christie, Trump would be doubling-down on the real mogul's own style of Republicanism: blunt, focused more on projecting strong leadership than policy positions, and downplaying social issues like same-sex marriage and abortion.
Hogan is in only his second year as Maryland's governor. But he has governed as a moderate and has a high approval rating in a generally liberal state. His mild-mannered style could be a helpful contrast, reassuring voters that a calmer presence will be working with the fiery Trump.
Gingrich, Graham and Rubio would in some ways resemble the Joe Biden model of the vice presidency. Biden struggled in his 2008 campaign with voters, but he is popular with Democratic insiders, from civil rights leaders to labor union heads. And much of a vice president's job is behind-the-scenes, shoring up support for a president's initiatives with members of Congress and interest groups. Graham has served in Congress since 1995 and has deep relationships with military leaders, the press and other Republican officeholders.
Rubio is also popular with insiders, as his long list of endorsements during his campaign illustrated. He is also a strong opponent of abortion rights and tax increases, while supporting a very hawkish foreign policy vision. His presence on the ticket could reassure conservative elites wary of Trump's commitment to party orthodoxy.
Gingrich of course ran the House during the 1990's, so he has a wealth of legislative experience. But during his 2012 presidential campaign, many of Gingrich's former colleagues expressed doubts about his temperament and judgment. A Gingrich selection may not be as popular on Capitol Hill as his resume suggests.
Trump, in an interview Wednesday on MSNBC's Morning Joe, indicated, as he has before, that he is not likely to pick a political outsider like himself as a running mate.
"I probably will go the political route. I have the business -- let's call it talents," Trump said. "And I think I'll probably go the political route, somebody that can help me with legislation and somebody that can help me get things passed and somebody that's been friends with the senators and the congressman and all, so we don't have to go the -- so we don't have to go the executive order route as much as Obama did, you know, where he can't get anything approved so he just keeps signing executive orders."
Biggest Potential Boost to Candidate's Image: Minnesota Sen. Amy Klobuchar, West Virginia Sen. Joe Manchin, Missouri Sen. Claire McCaskill, Vermont Sen. Bernie Sanders, Massachusetts Sen. Elizabeth Warren.
Clearest Path to 270 Electoral Votes: Ohio Sen. Sherrod Brown, Virginia Sen. Tim Kaine, Virginia Sen. Mark Warner
Biggest Potential Benefit to Governing: Labor Secretary Tom Perez, Massachusetts Gov. Deval Patrick
Wildcards: New Jersey Sen. Cory Booker, HUD Secretary Julian Castro, Mike Mullen, former chairman of the Joint Chiefs of Staff,
Critics say Clinton's candidacy is too establishment-oriented, safe and dull. Picking a female running mate could solve all those problems. The former secretary of state, generally a cautious politician, would be taking a really bold, aggressive step to change the face of American leadership. It would also be a direct challenge to Trump--who has accused Clinton of playing the "woman card" too often--and put gender even more at the center of the presidential campaign.
In choosing Sanders or Warren, Clinton would be signaling a more left-leaning, populist presidency than her husband's administration or Obama's. It would be an acknowledgement that she is wary of losing the Sanders' coalition and needs to ensure that bloc of the party turns out in large numbers.
Manchin, a centrist Democrat, would be the opposite move ideologically. The ex-West Virginia governor has argued at times that Obama is too liberal and has won repeatedly in a state that is overwhelmingly white and working class. He, unlike Clinton, might have strong appeal to the voters that Trump is targeting and who have backed Sanders over Clinton in the Democratic primary.
Since Mullen has spent much of his life in the Navy, it's not clear if he is even a Democrat. (He was head of the Joint Chiefs under Obama, but it is a non-partisan role.) That said, Clinton could opt to run a campaign focused less on ideological differences with Trump, instead arguing that the real estate mogul is simply unqualified to lead the country. A Clinton-Mullen ticket would have deep national security experience. And Mullen, who has never served in elective office, would help Clinton in appealing to those who like that Trump is not a career politician.
Again, the evidence is limited about the electoral boost that vice-presidential candidates provide. That said, Brown is popular in Ohio, despite a very progressive style of politics that is perhaps to the left of his state. Brown is similar to Sanders ideologically, and could appeal to his supporters who are wary of Clinton being too centrist.
Kaine and Warner are more moderate than Brown, but also have been very successful in a key state, both getting elected governor and then senator in Virginia.
Perez and Patrick are intriguing because of their long resumes in government. A Hillary Clinton administration, particularly if Republicans keep control of the House, is likely to try to use executive actions and work with left-leaning cities and states to accomplish its goals.
Perez and Patrick might be able to help implement such a strategy.
Patrick ran the Civil Rights division of the Department of Justice from 1994-97 and then served two terms as governor of Massachusetts. Perez's resume is even more diverse: top posts at DOJ and HHS in Bill Clinton's administration, a stint advising Ted Kennedy in the Senate, a brief career in elected office himself as a Montgomery County city councilman and now Labor Secretary under Obama.
Like Biden, both Patrick and Perez have deep ties to the civil rights community, labor and other key constituencies in the Democratic Party.
One of the most intriguing questions for Clinton is if she will feel pressured to put a person of color on the ticket. She is likely to have two African-Americans much more famous than Booker and Patrick campaigning across the country for her: Barack and Michelle Obama. The specter of a Trump candidacy may motivate Latino voters more than Castro or Perez being on the ticket.
On the other hand, Clinton has won the Democratic primary largely because of support from black voters and will need African-Americans and Latinos to overwhelmingly back her to win the general election. Does she need to pick a person of color to communicate she does not take minority votes for granted?
Castro (41) and Booker (47) also have the virtue of being relatively young. The 68-year-old Clinton has struggled with younger voters throughout the primary, although early polls suggest they prefer her to Trump.