There's good reason to be worried about going against Trump, given how popular he is among the Republican base. But I'd argue this explanation is at least somewhat incomplete.
A lot of GOP senators may be willing to vote with Trump because they aren't against him. They, like their voters, may really like him and his policies or believe he didn't commit an impeachable offense.
This theory is probably hard for a lot of Trump critics to take, but it does make sense. Trump retrospective approval ratings remain in the 80s with Republicans. A lot of Republican lawmakers are likely reflected in that high approval rating.
And by voting against conviction, these senators are ensuring that Trump is with the Republican Party for the foreseeable future.
Besides, if Republican senators were really worried about their future electoral ambitions, there are reasons to think many of them could survive a primary challenge even if they voted to convict.
Most Republican senators are either retiring or aren't up for reelection until at least 2024. In fact, at least 34 of 50 GOP senators are not running for reelection in 2022 at this point. (It could be more based on retirements.)
That means there would be at least three years until the senators who don't retire would face electoral consequences for any votes against Trump.
Three years is an eternity in politics. Don't believe me? Ask Senate Minority Leader Mitch McConnell.
McConnell's favorable ratings with Republicans nationwide plummeted from the 50s to the low 30s in 2017, after Trump and the Kentucky Republican tussled over their efforts to end the Affordable Care Act. After a little more than a year, McConnell's favorable ratings jumped back up into the 50s. He benefited from pushing through the confirmation of Brett Kavanaugh to the Supreme Court in 2018.
There are likely to be many more battles in President Joe Biden's administration. Politics isn't static. There's no guarantee that votes on impeachment will be the determinative factor in primaries well down the road.
Keep in mind that most Republican senators voted against objections to the Electoral College results in January. They did so even as their voters believe, falsely, that there was wide-scale election fraud.
Republican senators have shown a willingness to go against Trump. So the idea that they aren't willing to in this situation just because they fear him seems a little hard to swallow.
So, to me, a plausible possibility remains what I posited at the top: Maybe many GOP senators actually really like Trump or at least buy his arguments on impeachment.
Either way, Republican senators realize that Trump remains a powerful force within the party.
Trump's actually the favorite for the 2024 presidential nomination. He is easily leading all primary polls. As I previously noted, Trump is in the best position of any former one-term president in the polling era looking to reclaim his party's presidential nomination.
There is little reason to believe Trump won't try to take advantage of these numbers. He loves the limelight and the adoring crowds. Soon enough, Trump will probably be on the campaign trail.
The only lesson Trump is going to learn from this impeachment saga is that he controls the Republican Party. Republicans basically passed up their best opportunity to rid themselves of him.