Mr. Cruz won without Mr. Rubio’s endorsement, and later confided to a Republican senator that he “resented” Mr. Rubio’s reluctance to endorse him. Now, the two Republican stars, biographically similar but stylistically opposite, are running for president, and Mr. Cruz is privately telling colleagues that he believes the race for the party’s nomination will boil down to a contest between himself and Mr. Rubio…

Republican senators said that while Mr. Rubio and Mr. Cruz were never close, they had detected a chill between the two. “Their relationship has diverged,” Senator John McCain of Arizona said.

One senator used the word “wariness.” Another said that an unpersuasive argument for getting one on board with legislation would be pointing out that it was championed by the other…

But they are already competing in the Senate for the endorsement of Senator Mike Lee of Utah, who is admired by the conservative base for his anti-spending positions, and whose backing is considered a conservative seal of approval.