At the state convention on June 7, 2008, Edward O'Reilly received 22.5% of the vote, thereby granting him a spot on the September primary ballot. Kerry, received the vast majority of votes, however, granting him the convention's endorsement. On July 25, O'Reilly challenged Kerry to a series of debates, and the two eventually met for one debate in early September. On September 16, Kerry defeated O'Reilly in the Democratic primary.
Kerry had explored the possibility of seeking the Democratic nomination for President once again in 2008, a circumstance which prompted many of Massachusetts's all-Democratic House delegation, including Marty Meehan, Ed Markey and Stephen Lynch, to declare that they would run in the Democratic primary for Senate should Kerry not seek reelection. However, on January 24, 2007, Kerry announced that he would run for reelection to the United States Senate and not for the Presidency. A December 23, 2007, poll indicated his approval rating at 52%, with 43% disapproving.
Massachusetts held primary elections on September 16, 2008. Unlike in 2002, when Kerry was reelected against only third-party opposition, the Democratic nominee faced a Republican opponent in 2008. The Republican candidate, businessman Jeff Beatty who had lost a congressional race two years earlier, was not considered a serious contender in this overwhelmingly Democratic state. Massachusetts had not elected a Republican Senator since Edward Brooke in 1972 and has not sent any Republicans to Congress since incumbents Peter Torkildsen and Peter Blute were defeated in 1996.
On September 16, 2008, Kerry defeated Edward O'Reilly in the Democratic primary and faced Republican Beatty in the November general election.