This might just be the Best. Week. Ever. for Senate Democrats. At least so far this cycle.

Analysts originally believed Democrats would have a very hard time holding on to their three-seat Senate majority in 2012. After all, the math was against Democrats this cycle: 10 GOP-held seats are up in November, compared with 23 Democratic-held seats — many in Republican states.

While the GOP could still net the four seats they need to win an outright majority, the road to 51 certainly got tougher this week, write Kyle Trygstad and David M. Drucker on the front page of today’s Roll Call.

First, former Sen. Bob Kerrey (D-Neb.) decided to run for Senate after all. And Sen. Olympia Snowe (R-Maine) stunned Capitol Hill with her retirement announcement. Abby Livingston covers the continuing fallout from Snowe’s departure also in today’s Roll Call.

These boosts come following a few other lucky breaks, and recruiting successes, for Senate Democrats already this year. In the wake of these events, Roll Call is altering its race ratings for several Senate races — some of which are a little overdue.

The Senate race in Maine is now a Tossup following Snowe’s retirement.

Kerrey’s candidacy makes the open Nebraska Senate seat more competitive, but it’s still an uphill climb for Democrats. Roll Call moves this race from Likely Republican to Leans Republican.

In Michigan, former Rep. Pete Hoekstra’s (R) controversial TV ad set back his bid to take on Sen. Debbie Stabenow (D) — and the polls prove it. Roll Call moves this race from Leans Democratic to Likely Democratic.

In North Dakota, former state Attorney General Heidi Heitkamp (D) has proved she’s a good candidate for retiring Sen. Kent Conrad’s (D) seat (although it will still be hard for her to live up to her own hype touted by Senate Democrats). The GOP still has an advantage here, but Roll Call now deems this a Leans Republican race.

Finally, in Arizona, former Surgeon General Richard Carmona (D) has demonstrated  fundraising prowess and hired a strong team in the race for retiring Sen. Jon Kyl’s (R) seat. He still has a tough primary against former state Democratic Party Chairman Don Bivens, but momentum is clearly on Carmona’s side. Accordingly, Roll Call now rates this race Leans Republican.