Republican Mark Kirk of Illinois, possibly his party’s most vulnerable senator, not only opposes Donald Trump’s election as president. Kirk has run TV ads boasting that he has “bucked his party,” calling Trump “not fit to be commander-in-chief.”

The distance that Kirk and some other Republicans are seeking from the party’s presidential nominee — marked by their absence from the national convention this week — is a measure of fear that Trump may not only lose to Hillary Clinton in November, but also take the Republican-led Senate down with him.

Independent observers say Republicans have good cause for concern that, whomever wins at the top of the ballot this fall, the party could well lose its 54-46 majority in the Senate — and with that, any checks and balances the GOP might hold against potentially another Democratic president.

Two dozen of 34 Senate seats up for election this year are held by Republicans. Several seeking reelection, as well as one seat opening with a retirement in Indiana, are considered vulnerable by independent, nonpartisan analysts. And only one Democrat-held seat, that of retiring Senate Minority Leader Harry Reid of Nevada, is considered truly winnable by a Republican.

“It’s absolutely realistic,” Jennifer Duffy, senior editor for The Cook Political Report analyzing Senate races, says of a Democratic takeover this year. “If you only look at our tossup column (contests which polling portrays as too close to call), we have one Democrat and seven Republicans.”

In their nonpartisan report, Stuart Rothenberg and Nathan Gonzales predict: “Democrats are most likely to gain three to six seats.” With five, they can claim a majority. With just four, and possibly a Democratic president, the vice president could break tie-votes in a 50-50 Senate.

With a polarized electorate, Duffy says, Republicans in close contests have reason to worry that Democrats and independent voters stirred to oppose Trump will turn out in force and that Republicans who say they cannot vote for Trump will either pick a third-party candidate or simply stay home.

“Republicans are going to start talking about why a Republican majority is important, to serve as a check against whomever is elected, which is an interesting message to run about your own nominee’’ and his chances of winning, Duffy says. “They’ll be telling voters there’s no law that says you have to vote for president, and they’ll try some ticket-splitting.”

“The Democratic strategy is the same in all these cases: ‘The Republican candidate equals Trump.’” she says. “The question is whether voters believe that.”

Greg Blair, a spokesman for the National Republican Senatorial Committee, acknowledges the challenge but says his party is focused on making each race a question of voters’ concerns in each state rather than a national contest.

“We’ve known from Day One that this is a tough map for Republicans,’’ Blair says. “We’re defending 24 seats. Democrats set their sights on the majority really two Novembers ago, and obviously we knew we were in for a fight.”

Asked about nervousness about Trump, Blair says: “I don’t know I would characterize it that way, but I do know something our people are focused on is running their own race. Republicans are running for the Senate as if they were running for sheriff… We want to make sure that when voters cast a ballot in that race, it’s all about that contest, not up-ballot.”

Democrats are intent on tying senators and candidates to Trump. “Certainly we have launched a ‘Party of Trump’ campaign, since before he was the nominee, that really centers around the fact that these Republican candidates and senators not only have stepped aside by allowing Trump to be the standard-bearer of the party — but that the Trump agenda would be the Republican Senate agenda,’’ says Sam Lau, a spokesman for the Democratic Senatorial Campaign Committee.

Both parties are putting aggressive fundraising behind their messages, the committees holding nearly $30 million apiece at the end of June. Early polling suggests the hardest battle lines will be drawn in Illinois, Wisconsin, Indiana, New Hampshire and Nevada. Ohio and Pennsylvania also are close.

This helps explain why Kirk — who claims his own polling portrays a dead-heat with Democratic rival Tammy Duckworth in Illinois — is running ads boasting of being the first Republican to support a vote for President Barack Obama’s Supreme Court nominee, Merrick Garland, being “a leader on protecting a woman’s right to choose’’ and the first Republican senator to “buck” Trump.

Obama carried his home state by 17 points in the last election, and Clinton holds an 18-point advantage over Trump there in the latest public polling.

In Wisconsin, Republican Sen. Ron Johnson is in trouble, according to the University of Virginia’s Larry Sabato, echoing the views of other independent analysts in his “Crystal Ball” assessment. Former Sen. Russ Feingold, attempting a comeback, holds a 9.3 percentage-point advantage in the average of polls tracked by RealClearPolitics.

In Indiana, former Democratic Senator and Governor Evan Bayh’s recent decision to seek the open seat of retiring Republican Dan Coats has thrust that contest into a toss-up in both The Cook Political Report and The Rothenberg & Gonzales Political Report.

In New Hampshire, Republican Sen. Kelly Ayotte has been critical of Trump — sitting out this week’s convention in Cleveland — while ostensibly standing by the party’s nominee. Ayotte faces a challenge from another popular leader in the Granite State, Democratic Gov. Maggie Hassan. Polling portrays a dead-heat in a state also heavily contested by Trump and Clinton.

Trump “is dangerous to the country,” Hassan has told reporters. “I am appalled that Sen. Ayotte is supporting him, and I think she absolutely will need to be held accountable for Donald Trump’s statements and positions.”

As Democrats seek to nationalize these races, Republicans are turning them homeward — Ayotte campaigning on her concern about chronic opioid abuse, Republican Sen. Marco Rubio of Florida warming to the fight against the Zika virus. So long as he survives his own party’s primary — an Aug. 30 challenge by a Gulf-coast multimillionaire — polling suggests Rubio can withstand that of Democratic Rep. Patrick Murphy.

After initial hints that Democratic Colorado Sen. Mike Bennet might have problems, analysts agree the one best chance the GOP has to blunt any Democratic gains is in the home state of the party’s minority and ex-majority leader.

“Nevada is the top pick-up opportunity for Republicans, no question,” the NRSC’s Blair says. Democrats, in turn, acknowledge a competitive contest but see a strong contender in Catherine Cortez Masto, potentially the first Latina U.S. senator.

In Ohio, Republican Senator Rob Portman holds only a 2.7-percent edge over former Gov. Ted Strickland in the RealClearPolitics average of polls. Pennsylvania Sen. Pat Toomey holds a similarly narrow lead against Katie McGinty, a former environmental regulator who lost a governor’s race two years ago.

The Cook report considers these contests toss-ups, Rothenberg and Gonzales “toss-ups leaning Republican.” Republicans are banking on incumbents in both states, suggesting that Portman and Toomey are running model races — Portman recently out building homes with Habitat for Humanity — and rivals running “flat.”

While Democrats focus on several prime targets, they also see a “second tier” of possibilities in states such as Arizona, Missouri, Iowa and North Carolina. Yet the Republicans maintain it all comes down to the quality of individual candidates.

“Democrats are celebrating the fact that they have a lot of seats to work with,” Blair says. “But I think both of these races, Ohio and Pennsylvania, are emblematic of the problem they’ve had recruiting candidates for battleground states.”