From Emily Friedman and Arlette Saenz:

Florida Sen. Marco Rubio will campaign with Mitt Romney on Monday for the first time this election, ABC News has learned.

Romney and Rubio will hold an event in Delaware County, Pennsylvania on Monday, outside Philadelphia.

Rubio endorsed Romney in late March during an appearance on Fox News.

"It's increasingly clear that Mitt Romney's going to be the Republican nominee," Rubio said. "We've got to come together behind who I think has earned this nomination and that's Mitt Romney."

Rubio, who often appears high on lists of potential vice presidential candidates, said Thursday that he would turn down the offer if Romney asked him to be on the GOP ticket this fall.