Is Rubio gaining late momentum in Iowa?

But something appears to be shifting for Senator Marco Rubio of Florida. It is a change evident in his growing crowds; a string of endorsements; a friendly nod from a popular Iowa senator, Joni Ernst; and the cautious confidence he is starting to express.

“I’m very optimistic about the energy we’re gaining, the endorsements, the support and the turnout on a day like today,” Mr. Rubio told a crowd of more than 300 people who came to hear him speak here at a Best Western.

Over the weekend, Mr. Rubio won the backing of the state’s largest newspaper, The Des Moines Register. And on Sunday his campaign announced that Ms. Ernst would join him for a rally in Des Moines on Monday. While that does not amount to a formal endorsement, Ms. Ernst has no plans to campaign with any other candidate.

All along, Mr. Rubio and his advisers have insisted that grumbling about his relatively light footprint in Iowa would look silly in retrospect. But there is still a week left before Iowans vote — plenty of time for his backloaded strategy to backfire. And by the time the week is over it will be clear whether or not the Rubio campaign is experiencing slow-building momentum that would position the senator to peak right before the caucuses.