Back-to-back surprises _ Maine Sen. Olympia Snowe is retiring and former Nebraska Sen. Bob Kerrey is running again _ have given Democrats a burst of optimism that they can hold on to control of the Senate, a goal that seemed to be slipping from reach just weeks ago.

The decisions this week by Snowe, a Republican, and Kerrey, a Democrat, put two seats Democrats were prepared to write off in 2012 back into play. Those developments, combined with positive indicators for Democrats like an improving economy and President Barack Obama's higher standing in polls, have some sensing a shift in the environment.

"Things have changed so much that Democrats' prospects of keeping the Senate are at least 50-50 now, if not better than that," said Geoff Garin, a Democratic strategist and pollster.

To be sure, Democrats must overcome challenges to compete aggressively in both Maine and Nebraska. Time is running short to recruit a strong Democratic candidate in Maine, and Kerrey will be forced to answer for his years away from Nebraska.

Despite those hurdles, Democrats now see three clear pickup opportunities in 2012, in Maine, Nevada and Massachusetts. Party officials also hope races in Indiana and North Dakota could become competitive. Kerrey, a former governor, two-term senator and presidential candidate, gives Democrats a prominent candidate to put up for a seat they were prepared to concede when Sen. Ben Nelson announced his retirement and Kerrey initially passed on the race.

The overall playing field still favors Republicans, who are defending 10 seats to Democrats' 23 in 2012.The GOP needs a net gain of four seats to take back control of the Senate. Republicans said a week of positive developments for Democrats doesn't change the broader picture.

"Republicans have successfully expanded the map in states across the country and we're well positioned to make gains in November," said Brian Walsh, a spokesman for the National Republican Senatorial Campaign Committee, Senate Republicans' campaign arm.

Democrats' positive developments also come with plenty of caveats.

The latest Associated Press-GfK poll showed Obama topping 50 percent when matched against each of the four remaining GOP candidates for president and holding significant leads. A strong presidential candidate often can help elect Senate candidates and influence other down-ballot races. But Obama's improved standing is only likely to last as long as the economy continues to recover. A downturn could have a dampening effect on Senate candidates.

And while Snowe's retirement gives Democrats real hope in a state that wasn't even on the map a week ago, it's no lock. Democratic-leaning Maine has elected plenty of Republicans in recent years, including Gov. Paul LaPage, a tea party favorite. Democrats must scramble to find a top-tier candidate for the open seat with a March 15 filing deadline looming.

Then there's Kerrey, who announced his Senate candidacy Wednesday after declaring only weeks ago that he wouldn't pursue Nelson's Senate seat.

Kerry is a name-brand candidate with a long history in Nebraska, and his presence alone could mean Republicans will have to devote resources to a race that they were banking on winning a few days ago.

Kerrey himself has said he'll be an underdog.

Nebraska has grown more conservative since Kerrey left more than a decade ago. He will have to fend off a carpetbagger label, having decamped from Nebraska for a job as president of the New School in New York City after leaving the Senate in 2001.

"The fact that Kerrey was touted and press-worthy shows the state of affairs for Democrats," said Brian Nick, a Republican strategist. "In a normal playing field that would have been a big yawn."

But Garin and other Democrats said the individual developments were less important than an overall trend. He said Democrats have been building steam for weeks and this week amplified their progress.

"The environment's been changing slowly even before this, but this week obviously opened it up," Garin said. "It put two more seats back in play for Democrats _ in an election where every seat will count and any one seat can make the difference."

Rodell Mollineau, president of Democratic group American Bridge 21st Century and a former spokesman for Senate Majority Leader Harry Reid, echoed Garin's optimism.

"If you look at the numbers, where we are, and the places for potential pickups and the strength of our incumbents, I think we're in a good place now," Mollineau said. "I think we've been in a good place; people are just starting to recognize it. This week put us in an even better place."