Much has been made about whether Republicans will ever cut ties with President Trump over his attempt to hold up military aid to Ukraine to get political dirt on former Vice President Joe Biden. And although there are some signs of the party breaking with Trump, one important reason why we haven’t seen GOP senators break with Trump en masse is because many are up for reelection — in total, there are 23 Republican-held seats up in 2020.

And in five of these races, Republicans face particularly tough reelection battles. Election forecasters have rated these races as the most competitive Senate contests in the 2020 cycle, and in each race, the partisan lean of their states is less than 10 points more Republican than the country as a whole. In fact, in the case of Maine and Colorado, they actually lean slightly Democratic.1 These senators are especially vulnerable in a general election if Trump’s standing declines, but they’re also in a position where they can’t afford to alienate their party’s base.