MARION, Iowa — For months, it was the biggest question nagging at the Rubio campaign here: “Where’s Marco?”

His schedule was not packed with 16-hour days spent rolling across Iowa’s hinterlands in a bus. As an Iowan, you were probably more likely to see him on Fox News or hear about him visiting New Hampshire or South Carolina. Other candidates like Ted Cruz and Donald J. Trump had the support of more high-profile conservative leaders.

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.