Byron York

So Specter, 79 years old and apparently determined to serve another term in the Senate, is in a very vulnerable position. "Democrats have the blackmail card," the strategist explained. "In the past, the Republicans could say to Specter, 'We're going to challenge you in the primary,' and he could say, 'To hell with you -- I'll become a Democrat.' He can't say that anymore. If the Democrats say, 'We're going to challenge you in the primary,' he can't say, 'To hell with you -- I'll become a Republican.' They'll say, 'Noooooo, you won't.'"

As far as Republicans are concerned, it couldn't have happened to a nicer guy. More than a few in the GOP were gobsmacked when Specter explained his defection in nakedly strategic terms. "He made perfectly clear in a private conversation with (Senate Minority Leader Mitch) McConnell that his decision was made ... when his pollster came to him and said, 'You will not win the Republican primary,'" a top Senate aide told me. "So the decision to run as a Democrat wasn't because he wanted to leave the Republican Party, or because the party was mean to him." Specter said much the same thing in public; his decision reeked of sheer desperation.

So now, it's on to the new 60-vote, filibuster-proof Democratic majority (assuming Al Franken eventually wins in Minnesota). Even though there's been a lot of attention paid to the Republican Party's new powerlessness -- and it's true, the GOP is toast -- Specter's jump also creates new pressure within the Democratic caucus. With Specter, and 41 votes, Republicans had the power to stop anything, but only if all their members stuck together. That made the inclinations of moderates like Susan Collins, Olympia Snowe and Specter critical to the GOP's fortunes. Now, with just 40 votes, that doesn't really matter. Republicans can stick together and still not stop anything.

Democrats, on the other hand, now have the power to pass anything, but only if all their members stick together. That makes moderates like Ben Nelson and Blanche Lincoln critical to the party's fortunes. "From now on, the story is, 'Can the president keep his troops in line?'" the Senate aide told me.

And the newest Democrat, Arlen Specter, better keep his new colleagues happy. After all, his future is in their hands.

Byron York

Byron York, chief political correspondent for The Washington Examiner