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.”…

Putting a brave face on it, some Republicans like Law insist that at the very least, Trump’s constituency is devoted enough to be counted on to show up to the polls in November. By contrast, they say, Clinton’s failure to earn the support of millennials (who went for her primary opponent, Bernie Sanders, by a staggering 43-point margin) could result in a decline in youth-voter turnout that might swing some down-ballot races their way. In other words, the depth of Clinton’s support is as questionable as the breadth of Trump’s, or so goes the argument. As Law — whose American Crossroads has spawned an offshoot organization, One Nation, that has made large ad buys in the states with vulnerable Republican Senate seats — told me: “In our research, we’ve found that upward of 30 to 40 percent of voters currently have a preference but aren’t satisfied with that preference. So I think there’s risk on both sides.”

I asked Law if he thought the risks really were that comparable. After all, while Clinton’s current disapproval rate is a bleak 55 percent, Trump’s is 15 points worse. In a neck-and-neck Senate race, won’t the greater dislike of Trump make a difference?

“At this stage of the game, I’m not sure you can say that,” Law replied. Of course, at this stage of the game, American Crossroads is investing huge amounts in such races, running ads that have plenty to say about Hillary Clinton and nothing about Donald Trump.