When he was a candidate, Mr. Walker offered kind words for Mr. Rubio, musing publicly that the Florida senator would make a good running mate. During a Milwaukee radio interview Wednesday, Mr. Walker said he expects to endorse a candidate before Wisconsin’s April primary, but he offered few clues about whom he would support, other than expressing his distaste for Mr. Trump’s candidacy. Mr. Walker’s office didn’t make him available for an interview.

In Wisconsin, there has been substantial momentum from Mr. Walker’s political machine toward Mr. Rubio. Last week, Robin Vos, the Republican Wisconsin state assembly speaker, announced his support for Mr. Rubio, and he is hosting a fundraiser in his hometown of Burlington. Nineteen other Wisconsin state legislators who had backed Mr. Walker are now for Mr. Rubio.

“I’m fully ready to get behind Sen. Rubio,” said Mr. Johnson, whose family owns Johnson Timber Corp. He said he expected to see other Wisconsin donors back the senator before a Nov. 10 GOP debate in the state.