The Politico reported Libertarian candidate Bob Barr and Constitution Party candidate Chuck Baldwin will pick up some of the longtime Texas congressman's supporters.
"I would be very surprised to see many people going for (Democrat) Barack Obama," said Jesse Benton, Paul's campaign spokesman. "Barr will pick up some, but the majority will go Republican or stay home."
Paul finished second in a few state nominating contests and he raised $17.75 million in the last quarter of 2007 -- the most money of any Republican.
For some Paul supporters, the only way they would support McCain is if the Arizona senator shifts his positions on foreign and economic policy, said David Hart, Paul's Montana state coordinator.
"Unless McCain does make changes in his platform," including abandoning his support for the Iraq war and renouncing deficit spending, "I don't think (Paul supporters) will be voting for him," Hart said. "They will more likely be voting for the Constitution Party or Bob Barr."