I was going to post on another subject, but things turned out to be more complicated than I thought, so here is a stopgap. The Washington Post has a story about how Republican senate candidates are emphasizing their closeness to Donald Trump. Although it offers some qualifications, it says that a major reason is that "Trump enjoys enormous popularity among Republican primary voters." According to the most recent Gallup Polls, 85% of Republicans approve of the job Trump is doing. How impressive is that? Between June 1, 2017 and today, Trump's approval rating among Republicans has ranged from 79% to 87%. Between June 1, 2009 and April 2010, Obama's approval ratings among Democrats ranged from 81% to 92%; between June 1, 2005 and April 2006, George Bush's approval rating among Republicans ranged from 75% to 89%. That is, it's normal for a President to have "enormous" approval among people who identify with his own party.
Only 7% of Democrats say that they approve of the job Trump is doing as President, but that's also normal. Things get more interesting among Independents: Trump has ranged from 30% to 33% approval (currently at 33%). Obama ranged from 43% to 60% (in early April 2010 he was at 43%), and Bush from 23% to 45% (26% in early April 2006)
Overall, 41% approve of Trump's performance in the latest Gallup Polls, 47% approved of Obama at the equivalent time in his first term, and 37% approved of Bush at the equivalent in his second term. The difference in overall popularity was concentrated among Independents. Probably some of that is people switching from saying that they support a party to saying that they are independent. It could be my memory, but I don't recall that Democrats were making efforts to tie themselves to Obama in spring 2010 or Republicans making efforts to tie themselves to Bush in 2006--in fact, they seemed to be going the other way and emphasizing their independence and commitment to do what's best for their state, which is a sensible strategy when your president is not especially popular.
Why are candidates trying to move closer to an unpopular president? I think that they are buying into the story that Trump has a particularly strong connection to "the base." In my view, what actually happened was that once Trump got the nomination he benefited from party loyalty (and even more important, from dislike of the Democrats). I've had several posts (especially this one) noting that there is no evidence that Trump voters were unusually enthusiastic.