For Senate GOP, 2010 Losses on Top Of the 2008 Losses

Others in the GOP pointed out that the retirements -- coupled with defeats at the ballot box in 2006 and 2008 -- have created opportunities for a younger generation of leaders to step in and reshape the party.

Former congressman Rob Portman, an almost-certain candidate for the Voinovich vacancy in Ohio, has long been seen as a rising Republican star but had not had an obvious statewide opportunity until now. Ditto Marco Rubio, the young, Cuban former speaker of the Florida House who is expected to announce his candidacy for Martinez's open seat in the coming days.

"A true shift is underway," said Alex Vogel, a GOP lobbyist who closely follows Senate races. He added that protecting an open seat is always a challenge but "when you can recruit people like Rob Portman, that's a great challenge to have."

Not so fast, responded Jim Jordan, a former executive director at the Democratic Senatorial Campaign Committee who ran the group's independent expenditure effort in 2008.

Jordan called the recent Republican retirements a "morale killer, a momentum killer, and a massive recruitment and fundraising problem," adding that "I'll bet 10 to 1 that Voinovich won't be the last" GOP senator to announce plans to retire.

Staff writers Paul Kane and Shailagh Murray contributed to this report.