House Speaker Paul Ryan is not a big fan of Donald Trump — that much is clear. He refused to back Trump during the Republican primaries and then held out on endorsing him even after Trump became the presumptive nominee. But eventually, Ryan did get behind Trump. And now, even after video of Trump bragging about being able to commit sexual assault has come to light, Ryan said he won’t defend Trump, but he has refused to unendorse the GOP nominee. Trump responded by attacking Ryan on Twitter.

A reasonable person might ask why, if Ryan was reluctant to support Trump to begin with, the speaker of the House isn’t washing his hands of the whole Trump campaign.

But the answer is simple: Republican voters like Trump; Ryan risks losing the support of rank-and-file GOPers if he hits Trump too hard.

Trump is more popular among Republicans right now than Ryan is. In the most recent YouGov poll, for example, Trump’s net favorability rating (the percentage of respondents who rate him favorably minus the share who have an unfavorable opinion) among Republican primary voters was +36 percentage points. Ryan’s was just +16 points. Perhaps more telling is that Trump’s “very favorable” rating among this group is 34 percent, while Ryan’s is only 13 percent.

This means two things. First, Ryan really doesn’t have as much credibility among Republicans as Trump does. Second, if forced to choose, Republicans would almost certainly choose Trump over Ryan.

Indeed, we’ve already seen Ryan struggle when he and Trump tussle. Take a look at Ryan’s net favorability among Republican primary voters since the Iowa caucuses in early February. I’ve taken a five-poll rolling average of YouGov polls since that time.

Although the data is somewhat noisy (as we would expect when looking at a subpopulation), the pattern is fairly clear. Ryan is about as popular now as he was at the beginning of the primaries, but his favorability dipped to its lowest point right around the time he was refusing to endorse Trump after Trump wrapped up the nomination. Indeed, more Republican primary voters had a negative view of Ryan than a positive view in the YouGov poll conducted immediately after Trump became the presumptive nominee. It was only after endorsing Trump that Ryan regained some of his likability among Republican primary voters.

Should Ryan care about his popularity among Republicans nationwide given that he is only on the ballot in Wisconsin? The most immediate concern for Ryan is that, in a broad sense, he represents congressional Republicans. If Trump fans feel that Republicans in Washington aren’t backing Trump, it could cause them to withhold support from Republican candidates for the House and Senate. And if they get elected, these candidates could feel pressure from Trump-fan constituents not to back Ryan for re-election as House speaker.

Ryan might also want to run for president one day. Whether Republican officials in Washington like it or not, Trump voters are a large portion of the Republican base (that’s why Trump won). Ryan may need their support to win the nomination and the general election. Otherwise, he might end up … well, exactly where Trump is right now: As a GOP nominee bleeding Republican support.