Lieberman said that he's a strong backer of likely Republican presidential candidate Sen. John McCain, R-Ariz., but won't be his running mate, The New York Times reported Monday.

Lieberman, who has gained the scorn of many of his former Democratic colleagues in Congress by delivering harsh attacks on presumptive Democratic presidential nominee Sen. Barack Obama, D-Ill., said he was undecided whether he would speak at the 2008 Republican National Convention.

His swipes at Obama have mostly come on the topic of foreign policy in the Middle East, where Lieberman is considered a hawk in his pro-Israel and anti-Iran views. But he said that since the primary elections, Obama has begun to change.

He's shown a "remarkable change of position" on many issues, Lieberman told the Times, adding, "Senator Obama has really moved. Since he clinched the nomination a month ago, in my opinion he has altered and nuanced more big positions more quickly than I can remember any other presidential nominees."