A politically shrewd Senate Democratic staff member chatting about the future of health care negotiations stopped in midsentence late Tuesday afternoon as news flashed across his computer screen. "My God," he said. "Byron Dorgan is retiring."

It was a thunderclap moment in the politics of 2010, an unfortunate twist for Democrats already looking at a difficult election year. Dorgan, a veteran of three decades in Washington, suddenly turned his North Dakota Senate seat from one that Democrats had a reasonable chance of holding into a likely pickup for the Republicans.

Worse from the Democrats' viewpoint, Dorgan's move fed exactly the story line that Republicans have been pushing hard: Combined with the retirements of Gov. Bill Ritter in Colorado and of a number of incumbent House Democrats, Dorgan's decision looked to be part of a mass flight of vulnerable members of President Obama's party from a grim political battlefield.

But the retirement stories didn't stop there, and the paradox is that by the time all the exiting was over on Wednesday, Democrats had reason if not to smile, then at least to give a sigh or two of relief.

The Dorgan news was quickly followed by Sen. Chris Dodd's announcement that he would not seek re-election in Connecticut. His party's operatives were torn in their responses.

Dodd is as well-liked in Washington political circles as he has become unpopular in his home state, and in theory, at least, the professionals wanted to put politics aside and mourn the end of his career. ("The guy deserves some respect," lamented one of the party's top campaign warriors. "Do we have to do politics 24 hours a day? And I'm paid to do politics 24 hours a day.")

In practice, they were grateful Dodd withdrew. A seat once so promising for Republicans now seems safe for Democrats again as Connecticut Attorney General Richard Blumenthal, a longtime electoral winner, becomes the favorite.

Ritter's retirement was equally a blessing for Democrats. They have strong potential candidates to replace him (including Denver's popular Mayor John Hickenlooper and, possibly, Interior Secretary and former Sen. Ken Salazar). And Ritter's move might save Sen. Michael Bennet from a Democratic primary -- if his current challenger, former Colorado House Speaker Andrew Romanoff, now runs for governor instead.

Such calculations and countercalculations underscored how truly complicated the political map for this year's battle for the Senate has become.

Not even the most optimistic Democrats think their party can escape losing seats. But with so many states now unexpectedly in play, surprise Democratic victories could offset some Republican gains. On the other side, retirements -- not to mention the moves of a certain president and vice president out of the Senate -- have opened terrain for the Republicans that would normally be blocked.

All by themselves, Obama's victory and his appointments to his administration threaten four previously solid Democratic seats: Obama's old Illinois seat, Vice President Joe Biden's in Delaware, Salazar's in Colorado and Secretary of State Hillary Clinton's in New York.

Republicans can reel off four other states where they currently have a better-than-decent shot at Democratic seats: the newly promising North Dakota, plus Arkansas, Nevada and Pennsylvania. And if the country is really gloomy on Nov. 2, the GOP thinks it has a shot at California. 

Even this scenario would leave Republicans just short of a Senate majority, and Democrats are betting that they will easily hold New York and California, while hanging on to Nevada -- Senate Majority Leader Harry Reid has relatively unknown opponents and will have a huge bank account -- and Pennsylvania. Arkansas Sen. Blanche Lincoln confronts the most difficult terrain of any Democrat this year, but she may profit from Republican divisions.

Then there are the Democrats' wild cards: five Republican seats where some combination of strong Democratic candidates, divisive Republican primaries, or potentially weak GOP nominees offers a chance to offset losses.

In rough descending order of possibility, these include Missouri, New Hampshire, Ohio, North Carolina and Kentucky. Democrats can't bank on any of them, but just a win or two would buy their majority protection.

It's thus very hard to see how the Republicans can take over the Senate. But with North Dakota changing colors, the Democrats' map is not a happy one. If managing a barely filibuster-proof majority has been hell for the party's leaders, this now seems to be one burden they won't have to worry about next year.

(c) 2010, Washington Post Writers Group