“Right now, Rubio and Walker seem to me like they’re on a collision course”

The VP jabs are not all that lighthearted. For each man, suggesting the other could serve as his deputy is a not-so-subtle way of suggesting his opponent is not as seasoned as he needs to be. Walker partisans can emphasize Rubio is a first-term senator; their counterparts imply the Wisconsin governor is dangerously unfamiliar with foreign policy.

The jockeying for position is expected to be particularly intense, according to GOP insiders, because they hold some appeal to many different strands of Republican opinion.

“We know where [Sen. Ted] Cruz gets his vote: It’s narrow and it’s deep and it’s unlikely to change,” said GOP strategist Matt Mackowiak, alluding to the Texas Republican’s appeal to the right flank of the party. “With Walker, he gets some from the Tea Party, some from mainstream Republicans, some from evangelicals. With Rubio, I don’t think his base is ideologically narrow — it’s pretty broad. They compete with each other in that respect.”

Rubio has one important vulnerability unshared with Walker: While the Wisconsin governor is the favorite to win Iowa’s caucuses, Rubio is not the clear favorite there nor in the next three contests, in New Hampshire, South Carolina and Nevada.