As we enter the 2020 campaign season it is a virtual certainty that Robert Mueller's report (one way or another) will become public in 2019. Once that happens it's a certainty that Donald Trump will be impeached by the 116th Congress. This will put the 22 Republican senators up for re-election in 2020 in the position of either voting to convict the president and remove him from office, or face a "Blue Wave" like the one that flipped 40 seats, giving the Democrats control of the House of Representatives.
Without Trump on the ballot, two-thirds of midterm voters, who ushered in the Blue Wave, said they cast ballots in response to him (including in the key Electoral College states of Wisconsin, Michigan, and Pennsylvania, which narrowly went for Trump). Polling prior to General Mattis's firing, the stock market crash, Trump's shutdown of the government, and the president, in Iraq in front of our troops, calling us "suckers" for engaging in global alliances that protect us and promote democracy showed him with a disapproval rating fluctuating between 56 percent and 52 percent. In one poll (Harvard CAPS/Harris) after these occurrences, nearly 60 percent of voters said Trump should either be impeached and removed from office or formally censured. Having read his Happy New Year tweet which includes the words "HATERS AND FAKE NEWS", there's nothing to suggest that his behavior, his negative poll numbers, or the Wave will subside by 2020.
If they're thinking about it, Republican Senators have nothing to lose by convicting the president. It's predictable that Donald Trump will not be re-elected in 2020. He is a political dilettante who increasingly looks like King Lear rumbling around the castle (White House) murmuring, "I am a man more sinned against than sinning," i.e., tweeting "no collusion, I need a wall, I'm not responsible for anything." He is patently losing touch with reality. If impeached and convicted by the Senate, there would still be a Republican president in office through 2020, and Mitch McConnell (himself being primaried) would still be in charge of the judicial nominations Republicans so covet.