Mr. Rubio is set to inherit about two-thirds of Mr. Walker’s big-donor fundraising apparatus, according to a member of the Wisconsin governor’s national finance committee.

He’s already won the endorsement of one of Mr. Walker’s Georgia state co-chairmen. Mr. Walker’s New Hampshire state co-chairman, Cliff Hurst, joined Mr. Rubio’s campaign with the same title and role, and the campaign said Tuesday it had landed five of the Wisconsin governor’s Iowa county chairmen.

“I think Marco is the clear, logical choice of Walker supporters,” said Bob Pence, a developer who had planned to host a Walker fundraiser at his Washington home on Friday.

Mr. Pence held a fundraiser for Mr. Rubio in June and said he’d work now to steer his political network to the Florida senator rather than to both candidates as he had been doing. Those donors can tap their personal networks to encourage friends to write checks of $2,700, the federal limit for a single candidate.