Some might say that Mr. Rubio and Mr. Cruz’s support is enough to put them alongside Mr. Bush or Mr. Walker, the two candidates who have led the polls and have often been described as front-runners for the nomination. But Mr. Bush and Mr. Walker are front-runners in spite of their standing in the polls, not because of it.

They’re front-runners because the other candidates do not appear to have enough support from party elites to sustain a national campaign. Those other candidates do not have natural factional bases — like moderates for Mr. Bush, and conservatives for Mr. Walker — that give them clear opportunities to win early contests, or do not have the potential to build broad enough coalitions to win the nomination.

Historically, the polling numbers for Mr. Bush and Mr. Walker are weak with respect to their odds of winning the nomination, which is as much an indictment of the rest of the field’s viability as it is a testament to their potential strength. Mr. Rubio is perhaps the only candidate with a good chance to join them at the top.