Now the man who helped create an outsider movement in national politics, only to have it eat him alive by the co-opter of that idea, must decide which group among his fellow lawmakers to join. Will he stand with the hold-your-nose set, as Mr. Paul has done, and support Mr. Trump? Or join forces with “Never Trump,” as Senator Graham did on Friday, and publicly decline to get on board?

Or will he take the route of Mr. Rubio, in effect giving a non-endorsement endorsement, saying he will support any Republican nominee, but not explicitly name Mr. Trump?

“I think all of us will be interested to see what position Senator Cruz takes,” said Senator Jeff Flake, Republican of Arizona, who is more or less in the same place as Mr. Graham. “After he pretty much excoriated Trump on the final day of his campaign, it would be quite a turnabout if he were to support him now.”

Millions of voters are watching, but for now, he won’t say. Mr. Cruz “is currently scheduled to be back in D.C. next week, and that’s as much detail as we’re sharing right now,” his spokesman Phil Novack said on Friday.