The path for Democrats to win control of the House of Representatives is steep and complicated, but plausible, particularly if Donald Trump emerges as the GOP presidential nominee.
It's long been clear that either party could win the White House or the Senate this year. Republicans have a 54-seat majority in the Senate, but are defending six seats in states where Democrats are strong (Florida, Illinois, New Hampshire, Ohio, Pennsylvania and Wisconsin.) Democrats are trying to win the presidency for the third straight time, which both parties have failed to do since George H.W. Bush succeeded Ronald Reagan in 1992.
But the House has long been assumed safe for the GOP, until polling showed the depths of Trump's current unpopularity with the general election electorate. He is by some measures the most unpopular presidential candidate in modern times.
Republicans currently hold 246 of the House's 435 seats, compared to 188 for Democrats. In June, voters in Ohio will decide the replacement for former House Speaker John Boehner, who resigned last fall. The Republican candidate is heavily favored to win that special election, giving Republicans 247 seats.
Democrats will then need to gain 30 seats in this fall's elections to get a majority.
Such a radical shift in the makeup of the House is entirely possible in theory, since Democrats gained 30 seats in 2006, while the GOP won control of 63 in 2010.
In reality, Democrats winning the House is still highly unlikely this fall. If Trump becomes the Republicans' nominee, he is likely to gain support among GOP leaning-voters who have reservations about the real estate mogul right now but view Hillary Clinton or Bernie Sanders as too liberal.
In addition, Democratic voters tend to be clustered into major cities, so their votes are in some ways wasted in House races, where liberal lawmakers often run up huge margins. Republicans, in control of most state legislatures, often pack a few districts in each state with Democratic voters, leaving the remaining districts with GOP majorities.
Because of the way House districts are drawn, Mitt Romney himself finished ahead of Obama in 226 of the 435 House districts in 2012, according to a compilation from the Cook Political Report, even as the former Massachusetts governor lost by about 5 million votes nationwide. This was on Election Day in 2012, when young, minority, liberal-leaning voters who often don't cast ballots in midterms were out in full force.
Those dynamics mean that everything has to bounce the Democrats way for them to win the House.
Here's a closer look at that path:
1. Democrats need to win most of the 28 districts that backed Obama in 2012 but have a Republican member of Congress
The Democrats' simplest path to victory in the House is a strong turnout among the people who voted for Obama in 2008 and 2012. If Hillary Clinton or Bernie Sanders wins an area, it is very likely that those same voters will back the Democratic congressional candidate.
Ticket-splitting dropped to its lowest rate in decades in 2012, when only 6 percent of districts backed one party for president but the other for the House.
For example, Frank Guinta, a House Republican who represents the area around Manchester, New, Hampshire, won his seat in 2010, a strong year for Republicans, lost in 2012, then won again in 2014. He ran against the same opponent, Democrat Carol Shea-Porter, in all three races.
In this district, it seems clear that the national political conditions are affecting the race more than the individual candidates. Shea-Porter will be a favorite if Clinton or Sanders beat Trump or Texas Sen. Ted Cruz in New Hampshire.
Twelve of these 28 House Republicans were elected in 2014. That means they don't have the advantages of long-term incumbency, to build up a huge following in their districts. If the Democratic presidential candidate wins their districts, they are likely to be defeated as well.
But some of these 28 GOP House members will be strong, no matter who the Republican nominee is. For example, Rep. Peter King has represented the Long Island area in New York since 1993. Obama won King's district over Romney by 4 percent in 2012, according to Daily Kos, which has compiled the presidential vote in each congressional district.
But King has a well-established brand as a moderate Republican. He regularly blasts the Tea Party and has often slammed Trump in the last year. He is not a dogmatic party loyalist.
It is easy to imagine, if Trump is a bad candidate in the general election, King publicly suggesting that he will consider voting for Clinton, not Trump, particularly since King worked with Clinton when she was one of New York's senators.
Six of the 28 Republicans in districts that Obama won are in New York. (All but one of the six is outside of the New York City area.)
Running on a ticket with Trump may not actually be a killer for these New York Republicans. Upstate New York is a place where Trump is well-known and his appeal to blue-collar whites may be strongest. It's not clear Trump will be a worse candidate in upstate New York than Romney was in 2012.
In fact, those Republicans may be worse off if they are running on a ticket with Cruz, who mocked "New York values" last year.
The Democrats are very unlikely to win all 28 of these races, even if Trump is the nominee.
2. Democrats need to win seats in areas where the electorate is gradually moving left
Fewer people voted for Obama in 2012 than in 2008 (as a percentage of the electorate) in almost every area of the country. But the president won by a larger margin in 2012 in Miami-Dade County, likely because of both the growing Latino vote there and the shift of Latinos away from the Republican Party.
Particularly if Trump is the GOP nominee, Republican members of Congress from the Virginia suburbs outside of Washington, D.C., South Florida, Minnesota, and Nevada should be concerned. Early polling data suggests Trump is weakest among college-educated voters (Northern Virginia, Minnesota) and among Latinos (South Florida, Nevada).
In these areas, general demographic and political trends are already moving to the Democrats' advantage. Trump could accelerate a change that was already likely to happen.
3. Democrats need to get lucky
Every year, some House incumbents lose for reasons that are not part of national trends. Some go down because of scandals. Others simply aren't as popular back home as polling and other data would indicate.
For example, Utah's Mia Love is the only black female Republican in the House and was invited to speak at the Republican National Convention in 2012. But she barely won her race in 2014 in a conservative district that includes parts of Salt Lake City and is a top target for congressional Democrats.
4. Democrats need a big victory at the top of the ticket
If Republicans don't win the White House, it's important the candidate not lose by too much, if the party wants to keep control of one or both houses of Congress.
In 2008, Obama won by 7 percent nationally, such a landslide that he was the first Democrat in decades to carry North Carolina and Indiana. In such an environment, Republican House and Senate candidates lost essentially every close race.
In 2012, Obama won, but by 4 percent. That more narrow advantage meant that many House and Senate Republicans were able to survive.
This scenario is what worries Republicans about Trump, more than Cruz. The Texas senator would be expected to run a campaign similar to Romney's 2012 operation, winning the overwhelming majority of conservative-leaning voters while struggling with Democrats, particularly people of color. If Cruz were unsuccessful, it would likely be more akin to Romney's narrow loss than John McCain's resounding defeat, which came at the end of a very unpopular George W. Bush presidency.
Trump is more of a wild card, with the potential to turn off conservative-leaning women and independents and generate a huge turnout of blacks and Latinos to vote against him.
An NBC News/Wall Street Journal poll this week showed Hillary Clinton and Cruz effectively tied in a head-to-head match-up (Clinton was 46 percent, Cruz at 44 percent).
In that same survey, Clinton led Trump 50 to 39 percent in a hypothetical contest.