Last September, as the idea that the Trump candidacy was something more than an ephemeral novelty began to sink in, Ward Baker, the executive director of the National Republican Senatorial Committee, sent a memo to his senior staff. “Let’s face facts,” Baker wrote. “Trump says what’s on his mind, and that’s a problem. Our candidates will have to spend full time defending him or condemning him if that continues. And that’s a place we never, ever want to be.” Baker’s seven-page memo, which was leaked to The Washington Post, instructed candidates to “run your own race” and to “show your independence” from the man he described as a “mis­guided missile” — but at the same time, he cautioned, be mindful of the fact that “Trump has connected with voters on issues like trade with China and America’s broken borders.”

Even eight months before Trump effect­ively claimed the nomination, Baker grasped the uniquely fraught terrain the party’s unexpected standard-bearer had laid out before the Republicans who would be sharing a ticket with him in November. The connection between presidential nominees and so-called down-ballot candidates is historically clear, and has become more so as the country has become more polarized. That’s especially the case when it comes to the Senate. With (for the most part) increasing frequency since 1913 — the year the 17th Amendment was passed, designating that senators would be elected by popular vote rather than by state legislatures — voters have tended to elect Senate candidates who belong to the same party as the presidential candidate they favor.

Both parties still hold close the memories of the exceptional examples of this phenomenon, the bonanza years and the catastrophes. In 1980, Ronald Reagan’s victory over Carter was the crest of a tidal wave that also gave Republicans 12 Senate seats and control over the upper chamber for the first time in almost three decades. In 1964, Barry Goldwater’s support for “extremism in the defense of liberty” cost Republicans two-thirds of both the Senate and the House.

But Trump presents a much more complex weather system for his ticket-mates to navigate than either of these cases. His views are not wedded to a coherent ideological movement within his party (as Goldwater’s were), nor is his unpopularity a simple judgment on his record (as Carter’s was). Instead, Trump is a sui generis figure who must be accepted or rejected on his own terms, not artfully hedged around in the way politicians are accustomed to doing. And while Trump was undoubtedly the most popular Republican primary contestant in a field of 17, it’s still not clear how many of his opponents’ supporters will vote for their party’s pick on Election Day. For an at-risk Republican senator this fall, to back away from Trump is, by extension, to snub his millions of die-hard loyalists, the one group of party voters that is sure to show up on Nov. 8. But to go all-in for Trump is to take leave of your Republican bona fides and embrace life as a Trump Mini-Me — a gamble that not a single Republican senator up for re-election this fall appears to have the stomach for.

None of this seems to overly concern Trump. When I asked him recently whether the party’s maintaining its majority in the Senate meant anything to him, he replied: “Well, I’d like them to do that. But I don’t mind being a free agent, either.” Trump has shown similarly little interest in helping his party’s committees build the sort of war chests typically required in a campaign year. After winning the presidential nomination on a shoestring budget and with fewer paid staff members than the average candidate for governor, he has been visibly reluctant to help build much in the way of national campaign infrastructure, sending a clear message to his fellow Republicans: This fall, you’re on your own. As Ryan Williams, a strategist with the 2012 Romney presidential campaign, told me: “Traditionally, the nominee has a robust campaign that absorbs the R.N.C. effort and works in tandem with the down-ballot campaigns. We did that with Romney in 2012. This time around, there’s a complete void at the presidential level. Trump’s trying to play a game of baseball and hasn’t put out an infield.”

In addition to Kirk, there are five Republican incumbents running in states that Obama won in 2012 whose fortunes are now lashed to those of the Trump campaign: Kelly Ayotte in New Hampshire, Ron Johnson in Wisconsin, Marco Rubio in Florida, Pat Toomey in Pennsylvania and Rob Portman in Ohio. In May, Ayotte offered her “support” for Trump, but a spokeswoman quickly clarified to reporters that Ayotte “hasn’t and isn’t planning to endorse anyone this cycle.” Johnson supplied messaging advice to the Trump campaign during the Wisconsin primary in April and declared the day after Trump vanquished Ted Cruz on May 3 that “I am going to certainly endorse the Republican nominee.” Two weeks later, however, Johnson ratcheted down his endorsement to Ayotte-esque “support,” warily adding that he would “be concentrating on the areas of agreement with Mr. Trump.”

Rubio, meanwhile, has remained in a state of Trump-induced torment ever since his drubbing in the presidential primaries. Before announcing that he would run again for the Senate, Rubio said that he would be “honored” to help his party’s nominee, but later hedged, saying he did not expect to speak at the Republican convention on Trump’s behalf — and finally declaring he would not attend the convention at all. “I think that the Senate needs to fulfill its role as a check and balance on the president, no matter who it is,” he said last month. This was clearly intended to suggest that, if re-elected, he would not blindly do the bidding of a President Trump — a notion that has prompted belittlement from Rubio’s Democratic opponents. “What’s so funny about that premise is that Rubio’s the only Senate candidate we’re running against who has proven he’s ineffective at standing up to Donald Trump,” Sadie Weiner, the Democratic Senatorial Campaign Committee’s communications director, told me.