Lieberman will deliver a speech when Republicans gather in St. Paul, Minn., to nominate McCain for president, a party official told The Associated Press on Wednesday. The official requested anonymity because a formal announcement had yet to be made.
CBS News reports that Lieberman is scheduled to speak on Monday, Sept. 1, which is the first night of the convention. President George W. Bush, First Lady Laura Bush, Vice President Dick Cheney and California Governor Arnold Schwarzenegger are also slated to speak that night.
Lieberman's office declined to comment.
Lieberman, 66, caucuses with Senate Democrats. The four-term senator has angered many Democrats with his strong support for the Iraq war and for backing McCain's bid for the White House. He is considered a potential McCain running mate.
Four years ago, former Sen. Zell Miller, a Georgia Democrat, praised President Bush and mocked the Democratic ticket as weak on defense in a speech at the GOP's national convention.
As Al Gore's running mate in 2000, Lieberman became the first Jewish vice presidential nominee. His campaign for the Democratic presidential nomination in 2004 failed.
After a surprising loss to Ned Lamont in Connecticut's 2006 Senate primary, Lieberman defied Democratic leaders and ran as an independent in the general election. Top Democrats backed Lamont, a political newcomer, and Lieberman won support from the GOP, including his friend McCain.
Lieberman tends to vote with Democrats on most issues and is a longtime supporter of abortion rights, a stance that would rankle conservatives if he were McCain's running mate.
Not only has Lieberman campaigned for McCain, he has criticized Democratic candidate. Senate Democrats have been tolerant of his political straddling because he holds their slim political majority in his hands.
Lieberman departed Tuesday for a trip to the Republic of Georgia, Poland and the Ukraine. He is expected to return to Washington on Thursday night.