If you are eally a Trump supporter, you will vote for republican senators and reps so that he will have the support he needs in congress.
It is not that simple. We have seen that some of the other Republican candidates will do anything they can to undercut and work against Trump. Like the people signing the letter we are discussing, or the politicians who reneged on their vow to endorse and support the party's candidate. There is no point in re-electing a bunch of people who will be working, like the Democrats, against President Trump, should his campaign succeed in November.
In some cases it is better to have your enemy be obvious, and from another party, than be saddled with enemies within your own party.