In the first five months of the year, it seemed like a Senator decided to call it quits once a week.

All tolled, eight Senators — six who caucus with Democrats and two Republicans — said “sayonara” by mid-May. (Wisconsin Rep. Herb Kohl was the last to bow out on May 13.)

Since then? Silence.

A few months ago, it felt like the dam was about to burst on Senate Democrats — with an exodus of members in swing states that would make holding the majority virtually impossible.

Those leaks have since been contained and no new ones have sprung up — bad metaphor alert! — leaving Democrats with a tough but not impossible path to retaining the majority next November.

There’s little doubt when looking at the national map that Democrats have far more vulnerabilities than Republicans heading into 2012. (That’s what happens when you have an election like 2006 when, literally, every close race goes your way.)

Of the 10 races that make this month’s Senate Line, eight of them currently held by Democrats — and half of that number are open seats without an incumbent.

But, Democrats’ prospects in Nevada have brightened of late with a primary challenge to Rep. Shelley Berkley (D) dissolving. In Massachusetts the party seems ready to rally behind former Obama Administration official Elizabeth Warren . (Whether that’s a smart strategy is a point of contention even within the Democratic party.)

And races in potentially costly places like Pennsylvania and Minnesota — both states held by freshmen Democratic Senators — don’t appear to be materializing, meaning that the DSCC will be able to spread more of its resources to their endangered open seats rather than dropping millions in the costly Philadelphia media market, for instance.

It’s always important to note that surprises can and often do happen in politics and there could always be a retirement lurking around the corner that we aren’t on the radar right now. (Think Evan Bayh, who announced his retirement in February 2010.) But, for the moment, silence on the retirement front is golden for Senate Democrats.

Below is our look at the 10 Senate races most likely to switch party control in 2012. The number one race is — duh — the contest viewed as the most vulnerable to a party switch.

Kudos? Critiques? The comments section awaits.

To the Line!

10. New Mexico (D): A host of local unions just broke from their national counterparts to endorse state Auditor Hector Balderas , a sign that the primary between him and Rep. Martin Heinrich (D) could be fierce even as the establishment lines up behind the Congressman. On the Republican side, former Rep. Heather Wilson does not appear to have much to fear right now from Lt. Gov. John Sanchez. Even after loaning himself $200,000 in the second quarter, Sanchez raised only $312,000 — half what Wilson took in. (Previous ranking: 9)

9. Wisconsin (D): With the recalls now over, this open-seat race can begin in earnest. Former governor Tommy Thompson (R) got the ball rolling this week by announcing his campaign co-chairs. The next question is whether he gets a primary challenge. There remains a very real question about whether Thompson can survive a primary especially given his ties to President Obama’s healthcare bill. Top potential opponents include former congressman Mark Neumann and state legislators Jeff and Scott Fitzgerald. Democrats lost their strongest candidate when former Sen. Russ Feingold decided not to run. Liberal Rep. Tammy Baldwin is in the race and likely a primary favorite against Rep. Ron Kind who continues to weight the race. (Previous ranking: 8)

8. Massachusetts (R): Democrats finally landed their big fish, with Harvard law professor and former Obama adviser Elizabeth Warren launching an exploratory committee this week. Warren, while untried as a political candidate, is expected to be able to raise big money for the race and is a hero to the liberal left. But, she faces a serious and crowded primary, and Sen. Scott Brown (R) has made few political errors since winning a special election in January 2010. (Previous ranking: 10)

7. Ohio (D): Republicans are lining up behind state Treasurer Josh Mandel (R), who proved with his second-quarter fundraising that he’s a real contender for the seat held by Sen. Sherrod Brown (D). Democrats are also treating Mandel like the presumptive nominee by attacking his short record — he was elected to his current post in 2010 — and his opposition to the default-averting deal to raise the debt ceiling. Mandel now has to prove that he can do more than raise money. (Previous ranking: 7)

6. Virginia (D): With Jamie Radtke’s primary challenge to former senator George Allen (R) going nowhere fast, it now looks like a near-certainty that it will be Allen versus former governor Tim Kaine next fall. And that makes Virginia the home to the marquee Senate contest — in our humble opinion — of the 2012 cycle. Since this race is likely to be very close, what happens at the top of the ticket in the presidential contest will likely matter. If Obama can re-create his 2008 magic in the Commonwealth, it will help Kaine. If not, we could be looking at a return to the Senate for Allen. (Previous ranking:6)

5. Montana (D): Like Virginia, this is a fight between two skilled and well-known politicians. Sen. Jon Tester (D) won in 2006 thanks in large part to the foibles of then Sen. Conrad Burns (R) but has proven to be an able politician in the intervening five years. GOP Rep. Denny Rehberg has held statewide office in the Last Best Place for the better part of the last two decades. The more this race devolves into a partisan scrum with national implications, the better it is for Rehberg. Tester needs to keep the contours of the race within the boundaries of Montana to claim a second term. (Previous ranking: 5)

4. Missouri (D): Some Republicans are more bullish than others about beating Sen. Claire McCaskill (D). Most of that has to do with the fact that the field to face her is so far undistinguished and undefined. Rep. Todd Akin is raising the most money, but even he was far behind McCaskill in the cash dash. Former state Treasurer Sarah Steelman has struggled mightily to raise funds, but has support in the grassroots of the party. The wildcard is wealthy businessman John Brunner, a political newcomer who definitely has an opening and has lots of establishment Republicans excited about his prospects. (Previous ranking: 3)

3. Nevada (R): Democrats got a big break here when Rep. Shelley Berkley’s (D) primary opponent, wealthy businessman Byron Georgiou, finally dropped out of the race. Polls from both sides show the race between Berkley and appointed Sen. Dean Heller to be very close, and it will likely stay like that all the way through election day. (Previous ranking: 4)

2. Nebraska (D): State Attorney General Jon Bruning (R) remains the frontrunner for the Republican nomination but he’s had his stumbles, which could open the door for state Sen. Deb Fischer (R). Sen. Ben Nelson (D) has a terrific campaign team — Paul Johnson may well be the best campaign manager of either party in the country — but it’s still hard to see how the Democratic incumbent can get 50 plus one in a state as red as Nebraska in a presidential election year. (Previous ranking: 2)

1. North Dakota (D): Senator Rick Berg (R). Get used to saying it. (Previous rating: 1)

Aaron Blake and Rachel Weiner contributed to this report.