Paul said he likely won't vote for McCain.

(CNN) – John McCain is lining up quite a few major endorsements now that he’s the GOP’s likely nominee — but at least one Republican said Monday he won’t be backing the Arizona senator.

Speaking to the Chicago Tribune, Ron Paul said he'd only support McCain for president if the Arizona senator "has a lot of change of heart" on his Iraq war stands.

“I can not support anybody with the foreign policy he advocates — you know, perpetual war…I think it’s un-American, unconstitutional, immoral, and not Republican," Paul said.

The comments come two days after the Texas congressman definitely ruled out running as a third party candidate, and said he was phasing back his presidential bid to focus on his home district and the re-election race he faces.

"If I were to lose the primary for my congressional seat, all our opponents would react with glee, and pretend it was a rejection of our ideas. I cannot and will not let that happen," Paul wrote on his Web site Saturday.

Paul also acknowledged he was cutting back his presidential campaign staff, but said he will not formally bow out of the race before the convention.

– CNN Ticker Producer Alexander Mooney