Washington (CNN) - Moveon.org on Friday announced that the group's members are backing Rep. Joe Sestak in Pennsylvania in the May 18 Democratic senate primary. That puts the liberal activist group at odds with Organizing for America, the grassroots group that's part of the Democratic National Committee and that largely supports President Obama and backs Sen. Arlen Specter in his quest to retain his seat.
In an email to Moveon.org members in Pennsylvania, the group said that 67 percent of its followers in the state back Sestak.
The message quotes one member saying that "Sestak not only has a better chance of winning in the fall, but he's a Democrat out of conviction, not desperation." The email quotes another Moveon.org member as saying, "Arlen Specter is an opportunist and cannot be trusted to side with the Democrats."
Specter switched from the Republican to the Democratic Party last year. Critics claim he did so to save his senate seat.
Meanwhile, Organizing for America is working hard to ensure Specter's re-election.
"OFA folks recognize he has been a strong ally to the president on issues," OFA National Press Secretary Lynda Tran said in a message to CNN. In the primary's closing days, members are "talking to people on the phones and at the doors, reaching out to their neighbors and engaging people," Tran said.
Obama has endorsed Specter in the race.
With polls showing a tight race between Specter and Sestak, get-out-the-vote efforts by groups like Moveon.org and OFA will be critical for both campaigns.
A fresh Muhlenberg College daily tracking poll shows the candidates in a statistical dead heat.