Is there anything that this is comparable to? It's stunning to me just how fast this happened. Has any president seen sitting congressmen show public support for a primary challenge less than a year into his term?
None of them have actually said that they support a primary challenge yet. They just say that they're agnostic....that is, that they're not sure who they'll support yet. Usually, this question isn't even asked, as no one is contemplating it. No one bothered to ask sitting Democratic members of Congress back in 2009 if they would support Obama's re-nomination in 2012, but I'm guessing that if this question had been put to them, then many/most of them would have said that yes, they'd like to see Obama run for a second term. But I'm guessing that some would say something like "Why are you asking about an election so many years in the future?"
Of course, Trump invited this talk himself by launching his 2020 reelection campaign in 2017. No president has ever officially started their reelection campaign so early.