Early 2011 was an avalanche of good news for Republicans intent on regaining control of the Senate.

Early 2012 has been a reality check.

Sen. Olympia Snowe’s (R-Maine) retirement announcement Tuesday and former senator Bob Kerrey’s (D-Neb.) announcement Wednesday that he will seek a return to the Senate punctuated what has been a gradual rolling back of the GOP’s early momentum in the race for control of the Senate in 2012.

Sen. Olympia Snowe (R-Maine) speaks to media outside her office on Capitol Hill in Washington. Snowe’s retirement announcement this week caught the GOP off-guard and complicated its efforts to win a Senate majority in November. (Carolyn Kaster — Associated Press)

Republicans are still primed to compete for a Senate majority, which would require a four-seat gain, but the map looks significantly more difficult than it did a few weeks ago — to say nothing of the political environment itself.

The GOP’s plans to regain a Senate majority looked good from the outset this election cycle, with the playing field for 2012 including just 10 Republican-held seats and 23 Democratic ones — including several in red states.

Considering the GOP needed to gain only four seats to win a majority, it was pretty clear early on that it was very doable.

From there, things only got better for the GOP, with the retirements of Sens. Kent Conrad (D-N.D.), Daniel Akaka (D-Hawaii), Jeff Bingaman (D-N.M.) and Herb Kohl (D-Wis.) opening up previously untargeted seats. In addition, embattled Republican Sen. John Ensign (R-Nev.) resigned, giving Republicans renewed hope to hold his seat. All of a sudden, the GOP had legitimate chances to win upward of a dozen Democratic-held seats, while really having to defend just one or two.

Sen. Ben Nelson’s (D-Neb.) retirement in late December was just the icing on the cake for a GOP whose hopes of winning the Senate looked to have improved over the course of the calendar year, even as the political environment shifted from a Republican-friendly one to more neutral.

But the pendulum has now swung back — at least a fair amount.

This week, Democrats nabbed arguably their best pickup opportunity in the country when Snowe unexpectedly announced her retirement. And they avoided Nebraska becoming a lost cause when Kerrey changed his mind and decided to run after all.

Much like Nebraska, in North Dakota, Conrad’s seat appears to be less and less of a lost cause for Democrats, with former state Attorney General Heidi Heitkamp (D) looking the part of a formidable opponent for freshman Rep. Rick Berg (R).

Suddenly, an election in which it looked like Republicans would be guaranteed at least a two-seat gain looks like one in which they can’t count on anything.

What’s more, Snowe’s retirement and Elizabeth Warren‘s strong campaign in Massachusetts have given Democrats something they haven’t had all cycle — a couple of really strong pickup opportunities. And if Democrats can steal both of those seats, Republicans’ chances of taking the majority will be severely undercut.

“We still have a credible path to the majority, but it is going to be hard,” said one Republican, who was granted anonymity to talk candidly about the Senate landscape.

Republicans insist the Senate map still favors them — and it does, as most of the seats that are most likely to change control are still Democratic seats — but they do acknowledge that this week in particular has been a setback.

“It’s unfortunate to have to move Maine on to the competitive list with Massachusetts and Nevada, but Democrats are still defending 23 seats to Republicans’ 10, and numbers don’t lie,” said Scott Bensing, a former executive director of the National Republican Senatorial Committee. “I’d still rather be us than them.”

Added another former NRSC aide, Brian Nick: “Sen. Snowe’s retirement might equate to the Democrats getting a cavity rather than a root canal.”

Democrats argue that the recent events represent something larger than simply chess pieces shifting around on the board; they say the GOP brand and Republicans’ actions on social issues are pushing the Senate races in the Democrats’ direction.

They are also playing up their chances of potentially winning in Arizona and in Indiana if Sen. Richard Lugar (R-Ind.) loses his primary.

“Independent and even Republican voters are looking for leaders who won’t focus on divisive social issues but instead focus on jobs, the economy and the middle class, which is exactly what Democratic candidates are positioned to do,” said Matt Canter, a spokesman for the Democratic Senatorial Campaign Committee.

While it’s still clear that the map favors Republicans, it’s no longer a situation where they can get by by only winning in red states.

Early on, one could have made the case that all they needed to do was win in Nebraska, North Dakota, Missouri and Montana — all states that went for the GOP in the 2008 presidential race and are likely to again, unless President Obama wins in a blowout this year.

Today, it’s looking more likely that they will also have to pick off an open seat or two in a swing state like Virginia or a nominally blue state like New Mexico or Wisconsin.

Which of course is still very doable — polling in all of those states is very competitive, if not slightly favoring the GOP in recent days — but that’s a lot harder than having to just win Missouri and Montana.