Mr. Rubio has preserved his reputation among Republican primary voters while other elected officials have struggled. Only Mr. Carson scores higher on the question of which candidate GOP voters would consider backing, giving him room to grow his support. And the Florida senator is an early beneficiary of Mr. Walker’s decision to call it quits. During a two-day swing through Iowa, a number of Republicans who turned out to hear from Mr. Rubio said they were leaning in his direction now that Mr. Walker is out of the race. He’s also picked up a number of Walker donors and early-state organizers.

“I was really torn between Walker and Rubio, but when Walker dropped out, that was that; my mind was made up,” said Scott Lake, 60 years old, a retired police officer from Waterloo. “I know a lot of my friends feel the same way.”…

“America is so divided,” said Jace Brady, 24, a graduate student at the University of Iowa who attended a Rubio event in Cedar Falls. “More than any other candidate in the race, Marco Rubio is the one person who can bring people together.”