Trump’s big win in Rubio’s home state of Florida in March forced the Republican senator out of the presidential contest. Now that Rubio has reversed his oft-repeated claim that he would not seek a second Senate term, a big Trump loss may undo Rubio’s second 2016 campaign.

A Real Clear Politics polling average shows a three point Clinton lead over Trump in the state, but a Quinnipiac University poll released Tuesday showed Hillary Clinton opening a 47%-39% lead over Trump in Florida. Quinnipiac found the two presumptive presidential nominees in a virtual tie there in May.

That’s terrible for Trump, who faces a near-impossible electoral map if he loses Florida. But it’s also bad for Rubio. The senator may have briefly looked like the Republican frontrunner last winter, but today he is just one of several endangered Republican incumbents whose reelection chances will fall further as Trump’s numbers drop.

“Rubio’s Trump challenge isn’t any different than any other GOP incumbent in a competitive race,” said Jennifer Duffy, who handicaps Senate races for the Cook Political Report. “If Trump gets blown out in Florida, it will be a whole lot harder for Rubio to win. And like his colleagues, he will be answering for everything Trump says and does.”

GOP leaders convinced Rubio to run again because he offers their best hope to hold the Florida seat against probable Democratic challenger Patrick Murphy, a House Democrat. But even with Rubio in the race, Duffy still rates the race as a toss-up. So do other political handicappers.

This is reflective of an electoral trend: Ballot splitting, where voters pick a presidential candidate from one party and one or more congressional candidates from another, has declined for decades. About 30% of voters split tickets in 1972, versus about 10% in 2012. Turnout shoots up in presidential election years because voters are motivated to vote for one of the White House candidates. With few split ballots, the down ticket candidates win or lose based on their presidential candidate’s success.

Rubio is far from the only Republican who faces an uphill battle because of Trump. As the presumptive Republican nominee slips in national polls, he threatens to lose Senate battleground states and House districts by margins that even good candidates can’t make up from with the small portion of voters willing to divide their votes between parties. The problem for Republicans is less that voters will switch from Trump to Clinton, but that turnout will fall among GOP loyalists relative to Democratic turnout.

Republican Senators Mark Kirk of Illinois and Ron Johnson of Wisconsin are underdogs this year because they are up for reelection in states Clinton is expected to carry easily. Little wonder that Kirk this month became the first GOP incumbent to rescind his Trump endorsement.

Sen. Kelly Ayotte of New Hampshire faces attacks over her votes opposing gun control measures, but her biggest problem is Trump’s uphill odds in New Hampshire, which has trended blue in presidential races.

Quinnipiac showed Clinton and Trump tied at 40% in Ohio. That’s okay for incumbent GOP Sen. Rob Portman, who has a strong campaign organization and more money than his challenger, former Gov. Ted Strickland. But that poll showed Trump dropping by four points since last month. If that trend continues, the gap at some point will outmatch Portman’s means.

The story is similar in Pennsylvania. GOP Sen. Pat Toomey may be able to handle challenger Katie McGinty if the presidential hopefuls maintain their current virtual tie in the Keystone State. But a bigger Clinton win could sink Toomey.

Democrats hope that if Trump slips far enough in the polls, challengers to Sen. Chuck Grassley of Iowa, Richard Burr of North Carolina, and John McCain of Arizona will have a chance. Arizona has an estimated 350,000 unregistered Latino voters Democrats are scrambling to sign up, hoping to capitalize on Trump’s attacks on Mexicans and a federal judge of Mexican heritage. Democrats argue registering a chunk of those voters could put the state in play for Clinton and buoy the hopes of McCain’s challenger, Democratic Rep. Ann Kirkpatrick.

Trump’s woes explain why oddsmakers currently like Democrats’ chances of regaining a majority in the Senate, where the GOP now holds a 54-46 edge. A Clinton landslide might even threaten Republicans 247-188 House majority. One analyst recently guessed that Clinton winning by eight points or more nationally would put the House into play

Of course, same factors that might hurt vulnerable Republicans if Trump flounders would help them if Trump performs well.

“If Trump won Florida, Rubio would almost certainly win,” Kyle Kondick, managing editor of Sabato’s Crystal Ball, wrote last week.

Trump’s recent performance, however, leaves members of his party worrying far more that he will hurt them than hopeful he will help. Even Rubio—who once questioned Trump’s competence, honesty, and anatomy—has his political fate tied to the businessman. That is a dangerous position.