Rubio Move Part of GOP Strategy To Keep Senate Even if Trump Loses

Marco Rubio's decision on Wednesday to run for re-election for his Florida U.S. Senate seat, at the urging of party leaders, is part of an aggressive series of moves the GOP is taking in the hopes of keeping control of the Senate, even if Donald Trump is badly defeated in the presidential race.

Key party officials, led by Senate Majority Leader Mitch McConnell, passed over several Republicans who were already running for the Florida seat to implore Rubio to seek a second term, even though the former presidential candidate had spent months saying that he was leaving the Senate.

Former President George W. Bush, who has largely stayed out of politics since he left office, is holding fundraisers for Republican Senate candidates in close races, even as his aides have said that he will not attend the GOP convention in Cleveland that will nominate Trump. And groups affiliated with the conservative Koch brothers are already investing heavily in campaign ads and ground operations to win key Senate races in states like Ohio and Wisconsin.

Rubio on why he's running for re-election 8:45

The Koch groups, like Bush, are not yet behind Trump

Republicans, with a 247-188 seat advantage, are heavily favored to keep control of the House. The party currently controls 54 Senate seats, but must defend six in states where President Obama won in 2012 and Hillary Clinton is favored (Florida, Illinois, New Hampshire, Pennsylvania, Ohio, Wisconsin). A recent Marquette University poll showed Wisconsin Sen. Ron Johnson trailing Russ Feingold, his Democratic challenger, by 9 percent.

"It’s a tricky election. It would have been no matter who the nominee for president was. We have 24 seats up. The Democrats only have 10, and you mentioned all of the places where we expect to have tight races – New Hampshire, Pennsylvania, Ohio, Wisconsin, Illinois, Nevada and Florida. And therein lies the majority," McConnell told conservative talk show host Hugh Hewitt in a recent interview.

RELATED: Rubio's Run is a High-Risk Move

For Republicans, Senate control is critical, particularly if Clinton wins. By running the Senate, Republicans can either block Clinton’s appointments for court nominations--including the U.S. Supreme Court--and attempts to fill key administration slots, or force Clinton to eschew more liberal picks for moderate or conservative ones. With Senate control these last

two years, Republicans have effectively halted Obama’s nominations and prevented Supreme Court nominee Merrick Garland from even getting a hearing.

"Control of the Senate may very well come down to the race in Florida. That means the future of the Supreme Court will be determined by the Florida Senate seat. It means the future of the disastrous Iran nuclear deal will be determined by the Florida Senate seat. It means the direction of our country’s fiscal and economic policies will be determined by this Senate seat. The stakes for our nation could not be higher," Rubio said in a statement explaining his decision to seek a second term.

Here’s a closer look at the key races for each party’s quest for a majority in the Senate:

The Most Vulnerable GOP Seats: Florida, Illinois, New Hampshire, Pennsylvania, Ohio, Wisconsin

In all six of these states, Republicans have incumbents who were first elected to the Senate in 2010. That was one of the strongest electoral years for Republicans in recent memory, with the party winning races across the board as voters expressed dissatisfaction with both the economy and Obama.

So it’s not clear if those who won these Senate races in 2010 are great candidates or just were elected in a wave year that gave them a huge advantage.

Compared to midterms, like 2010, presidential elections tend to draw more diverse voters, which favors Democrats in many of these states. And the vast majority of voters back the same party for president and the Senate. So Illinois, Pennsylvania and Wisconsin, states where the Democratic candidate almost always wins the presidential election, present huge challenges for Republican Senate hopefuls.

The Chicago-bred Obama carried Illinois by about 17 percent over Romney in 2012, but even John Kerry won the state by double digits in 2004.

McCain Walks Back Comments that Obama 'Responsible' for Orlando Attack 2:19

Illinois Republican Sen. Mark Kirk has been a harsh critic of Trump, saying he will not vote for the real estate mogul and calling him “racist” and “bigoted.” But Kirk is still an underdog against Democrat Tammy Duckworth, a U.S. House member who lost both of her legs during her service as a pilot in the Iraq War. (A rocket-propelled grenade hit a helicopter she was flying in.)

Ohio, New Hampshire and Florida are states where Republican presidential candidates like George W. Bush have won.

But Florida in particular will test the Trump effect. Does the man at the top of the ticket inspire a huge turnout of Latino voters eager to vote against him? And do those Latino voters then also vote out Rubio, even though he is a well-known incumbent in the state and Cuban-American?

Ohio’s Rob Portman has taken some moderate stands. Back in 2013, he announced his support for same-sex marriage, becoming one of the first Republicans in elective office to do so.

New Hampshire’s Kelly Ayotte, on Monday, joined the Democrats in backing a provision that would have barred people on the terror watch list from buying guns. She and Kirk were the only Republicans to back that measure, which failed.

One advantage Republicans have is that incumbents are running in all of these seats. Incumbents tend to be able to raise more money than other candidates. They also have both the name recognition and the campaign experience from their previous run. Getting Rubio to run in Florida was a coup for Republicans, even if he is not a guaranteed winner.

States Where An Anti-Trump Wave Could Lead to GOP Defeats: Arizona, North Carolina

John McCain is one of the most famous members of the Senate, has been elected five times and was the Republican nominee for president eight years ago. But now, he has several opponents in a Republican primary who are casting him as too liberal. If he wins that primary, McCain will likely face Democrat Ann Kirkpatrick, who is attacking McCain as too conservative and linking him to Trump.

McCain is in a complicated position. Trump overwhelmingly won the Arizona primary in March. The state’s conservative activists are very wary of immigration, and already have suspicions about McCain, one of the writers of an unsuccessful 2013 bill that would have granted citizenship to undocumented immigrants. At the same time, Arizona has a growing population of Latinos, who will be 26 percent of the state’s eligible voters in this November’s general election, according to U.S. Census data. McCain likely can't afford to turn off those voters by strongly embracing Trump's positions.

North Carolina is a traditionally Republican state. But it’s last two presidential elections were extremely close, with Obama winning by less than 1 percent in 2008 and losing by 2 percent to Mitt Romney in 2012.

And the state is in the midst of a radical reshaping of its politics, with North Carolina's urban cores (Charlotte, Raleigh-Durham) becoming increasingly liberal and at war with the Tar Heel State's more rural parts. The controversy over the so-called bathroom bill in North Carolina was really a fight between the liberal Charlotte city government (which passed a law allowing transgender people to use bathrooms that correspond with the gender they identify with) and the conservatives who run the state’s government. Republican Gov. Pat McCrory, who signed the state law that invalidated the Charlotte provision, is also on the ballot this fall.

So incumbent GOP Sen. Richard Burr must figure out how to distance himself from the controversies of both McCrory and Trump without also irritating the state’s conservative voters.

Where Dems Could Lose: Nevada and Colorado

Democrats barely won these seats in 2010, and Nevada Democratic incumbent Harry Reid is now retiring. Republicans are running a strong candidate in Nevada, Joe Heck, a doctor and Army Reservist who was deployed to Iraq and served there in 2008. The Democratic candidate, former Nevada Attorney General Catherine Cortez Masto, would be the first Latino woman ever elected to the U.S. Senate. (Cortez Masto’s father is Mexican-American, her mother Italian.)

Democrats are favored in Colorado, with incumbent Michael Bennet seeking reelection. Nevada is considered a true toss-up.

It adds up to a difficult map for Republicans, which is why Rubio's decision to seek reelection is a sliver of good news for a party looking to hold onto its Senate majority.