Republican presidential hopeful Scott Walker. (AFP/Getty Images/Frederic J. Brown)

A top donor backing Scott Walker said Monday that he gave money to four of Walker's Republican rivals after last week's debate, the latest sign of trouble for the Wisconsin governor's struggling effort to stay in the mix for his party's nomination for president.

Minnesota media mogul Stanley S. Hubbard told The Washington Post he has donated money to the campaigns of former Hewlett-Packard chief executive Carly Fiorina, Sen. Marco Rubio (Fla.), retired neurosurgeon Ben Carson and New Jersey Gov. Chris Christie. Asked how much he contributed, Hubbard would only say that he gave "generously."

Hubbard said his decision is not a sign that he has given up on Walker, who he continues to support. He likened it to investing in multiple companies on the stock market. He said it reflected his belief that there are several talented candidates in a highly volatile Republican primary.

"I'm smart enough to know there could be big changes," he said.

Hubbard said the only super PAC he has given to is one supporting Walker.

Walker has fallen badly in the polls in recent weeks -- one survey released Sunday showed him under one percent -- and he has faced pressure from some backers to make staffing changes.

Hubbard sees another challenge Walker faces: communicating his message.

"He's got to figure out how to come across on television," said Hubbard.

A Walker spokeswoman did not immediately respond to a request for comment.