A victory for Ryan — and Trump — would jump-start the legislative effort to replace Obamacare, the issue that propelled Republicans to the House majority in 2010. It would also allow Republicans to move forward with a major tax reform package, another top priority for both the president and speaker.

And it would signal that their uneasy partnership can work — at least for now.

If he fails, Ryan will face a fierce backlash, one that could “mortally wound” his ability to be speaker, in the words of a close ally. He likely wouldn’t face a challenge for his job, but also couldn’t count on much support from Trump, who doesn’t like to lose and hasn’t forgotten the criticism Ryan leveled at him during the presidential campaign.

Bottom line: The rest of the GOP agenda will be endangered if Ryan loses Thursday’s vote. The House majority would likely be at risk from emboldened Democrats. And Ryan can expect a fresh wave of criticism from conservative hard-liners, who have already turned against a man they backed for the job.

Ryan and his allies believe he’ll win but that it will be razor close.