In other words: Trump is Senate Republicans' worst nightmare right now. His ban-Muslim-immigrants comment is undoubtedly going to make it harder for vulnerable Senate Republicans — who are trying to hang onto the Senate by defending 24 of the 34 seats up for reelection in 2016 — to argue that their party hasn't gone off the deep end ideologically.
We used the Cook Report's analysis of the most vulnerable Senate Republicans this cycle to rank who has the most to lose should Trump more permanently poison the well in the 2016 election — along with what the senators are saying.
1. Illinois Sen. Mark Kirk
Cook ranks the first-term Kirk as the most vulnerable incumbent seeking reelection; Obama carried Illinois by 17 points the last presidential election.
Which means the Republican Party isn't exactly popular right now in Illinois. And this was before Trump stole the party's spotlight with his not-so-inclusive remarks about women, Mexican immigrants, Syrian refugees and now, apparently, all Muslims, casting the party in a potentially negative light among already skeptical Illinois voters.
"If Kirk is to win a second term, the race really can’t be about him or the Republican Party," wrote Cook's Jennifer Duffy.
"If [Trump is] our nominee, the repercussions of that in this state would be devastating," said former state Republican chairman in Illinois, Pat Brady, in a New York Times article.
How Kirk has talked about Trump: In July, he said in an interview he'd tell Trump to "callate" —the Spanish version of "shut up."
2. Wisconsin Sen. Ron Johnson
Like many senators on this list, Johnson is fighting for a second term in a state that leans Democratic in a presidential year: It went for Obama in 2012 by seven points.
What's more, Johnson is one of the more conservative candidates up for reelection. At times, he's made some comments that sound like they could come from the mouth of Trump. On a local radio show in July, he sarcastically referred to "idiot inner-city kids" when making a case about school choice — a comment he said he regretted.
Johnson himself has drawn a parallel with Trump, as liberal super PAC American Bridge 21st Century pointed out in an August ad:
"I think what’s resonating about Donald Trump, I’d like to think, some things [sic] is appealing about my candidacy here in Wisconsin," he told a voter, according to the video.
As Trump gets further out there with his comments, Johnson might regret saying that, too.
How Johnson has talked about Trump: In September, he told voters Trump's plan to deport all 11 million undocumented immigrants was "not a solution."
Even though Portman has been in Congress off and on since 1993, he doesn't have that great of name recognition in his home state, especially compared to his challenger, former Democratic governor Ted Strickland.
Portman has 11 months to fix that. But to the extent that "R" next to his name is associated with Trump and not Portman's own well-regarded political brand, that doesn't help.
How Portman has talked about Trump: In September, he was one of a handful of GOP senators interviewed by the Hill who said he "intends to" support Trump if he were the nominee. "I guess you could come up with some scenario where something crazy could happen," Portman said, "but I think the country’s in trouble and I think if we don’t have new leadership and new policies, it’s hard to imagine it getting back on track.”
4. New Hampshire: Sen. Kelly Ayotte
Ayotte's reelection battle promises to be one of the closest Senate races of 2016. She's facing Democratic New Hampshire Gov. Maggie Hassan in a swing state that has leaned Democratic in recent presidential election years.
Ayotte's voting record has gotten more conservative as her reelection year nears, and Duffy writes Democrats' game plan has been to cherry pick her conservative votes to paint her as a more conservative candidate than she really is.
Aiding in that effort could be a controversial GOP presidential nominee.
How Ayotte has talked about Trump: She declined to answer reporters' questions Monday about whether Trump's comment hurts the Republican Party, according to Politico:
"You'll have to talk to the political pundits about that," she told reporters. "My position is there shouldn't be a religious-based test. We should conduct the same analysis for everyone's background, except for the added requirements that I've called for on the Syrian refugees given our inability to have all the information we need to vet."
5. Pennsylvania Sen. Pat Toomey
Right now, the first-term senator has the edge in his reelection race, especially since Democrats are still meting out who will be their nominee.
But like many of the states on this list, Pennsylvania tends to support Democratic presidential candidates.
"That means Toomey will have to outperform the GOP presidential nominee if he has any shot at another term," writes Duffy.
That could be tough when Democrats can dig into Toomey's resume to try to tie him more closely to Trump's brand of politics: He was the head of Club for Growth, an anti-tax, outside group known for targeting establishment Republicans in primaries — in Toomey's words, candidates that "ruffle feathers within the party establishment."
One plus for Toomey: The Club for Growth is no friend of Trump. The group's super PAC ran a TV ad in September claiming Trump supports higher taxes and currently has this on the home page of its website:
(Trump has threatened to sue for libel.)
How Toomey has talked about Trump: Toomey unequivocally maligned Trump's ban-Muslim-immigrants proposal in a tweet: