A know a woman who ran for Senate in Nebraska. She was raised in a small town in rural Nebraska. Her father is a Republican, and she was running for the Dem nomination. He said he would vote for her should she win the nomination, but he wasn't going to change his registration to Dem to vote for her in the primary. He had been registered for x number of decades, and continuing that streak was more important to him than supporting his daughter, who he claimed he preferred over the Republican Senator (Sasse). This is about teams, not policy.
The thing about an early development of a vaccine is that it is completely out of line with everything the President has been pushing. If he had told Americans to sit tight for a year, minimize contact, and hold out for a vaccine, then it makes sense that he would be vindicated. Instead, he told people it would disappear on its own. He was clear that the economy was more important than fighting the disease. I don't know if he ever said it directly, but certainly others in the Republican Party pushed the idea that the only path through was just to go on as normal, and eventually herd immunity (from everyone catching it) would cure it. The sooner a vaccine is developed, the more evident that this malpractice has costed 10's of thousands of lives that could have been saved had we just put a greater priority on slowing the virus.
I don't expect Trumpstains, or even the casual voter, to see it that way. I'm sure that a quick vaccine development will reduce fear, which helps the incumbent. But, for fucks sake, they killed a bunch of people do just to get a limping economy to move a little faster.