Twenty Senate seats have changed hands since 2006, the most competitive back-to-back-to-back election cycles since the 1940s. And it might only get more competitive in 2012.

Sen. Scott Brown (R-Mass.) waits for a hearing of the Senate Federal Financial Management, Government Information, Federal Services and International Security subcommittee hearing at historic Faneuil Hall in Boston in June. (AP Photo/Stephan Savoia)

Already, the Cook Political Report lists 10 Senate races as toss-ups — more than at this point in the 2010, 2008 or 2006 elections. Cook also rates 21 races as being at least somewhat competitive at this point, which is at least five more than any of the three preceding elections.

And if the Senate is indeed at stake — Republicans need to gain three seats for a tie and four for the majority — it appears as though it won’t be decided in just a couple states, but rather by competitive races all over the country.

In recent weeks, the 2012 map has taken a turn for the competitive.

Two of the seats that have long been at the top of the Line — North Dakota and Nebraska — both look like they are something closer to toss-ups today, especially if Nebraska Sen. Ben Nelson (D) seeks reelection.

Meanwhile, other states that don’t appear on the Line are starting to look even more competitive. Republicans have new hope in states like Florida, Hawaii and Michigan, while Democrats say they could have a third bona fide pickup opportunity in Arizona and some hope in Indiana if Sen. Richard Lugar (R) loses his primary.

In any other year, all or most of those states would have a good chance at making the Line. But the top 10 Senate races this time around are genuine toss-ups at the moment.

So what does it all mean? Practically, it means the national parties and outside groups could have a lot of races competing for their attention.

If so many toss-ups remain on the map, Republicans will again have to decide whether to go after the cheaper states or the states that appear the most winnable. For example, are they going to go to great lengths to defend Sen. Scott Brown (R-Mass.) in the Boston media market, or try to win an open seat New Mexico, which is much cheaper the play in?

Translation: it should be a fun cycle. And we shouldn’t lose track of the Senate races even as the presidential race heats up.

To the Line!

10. New Mexico (Democratic-controlled): Democratic Senatorial Campaign Committee Chairwoman Patty Murray (Wash.) made very clear recently where her party stands in the contested primary between Rep. Martin Heinrich and state Auditor Hector Balderas. “We recruited Rep. Heinrich, and we think he’s doing a great job,” she said at a recent press briefing. While that statement won’t make some in the Hispanic community happy, Heinrich is the more proven commodity and, at least according to recent polling, the stronger nominee against former congresswoman Heather Wilson, the heavy favorite in the GOP primary. (Previous ranking: 10)

9. Wisconsin (D): This is perhaps the most wide-open and interesting primary of the cycle on the GOP side. Former governor Tommy Thompson (R) has problems on his right, but he just got the endorsement of former Arkansas governor Mike Huckabee, which should help (to what degree, we don’t know). Former congressman Mark Neumann, meanwhile, has got the backing of tea party-aligned Sens. Tom Coburn (R-Okla.), Rand Paul (R-Ky.) and Jim DeMint (R-S.C.). And to top it all off, the National Journal recently argued that state House Speaker Jeff Fitzgerald also has a path to victory. The winner gets Rep. Tammy Baldwin (D). (Previous ranking: 9)

8. Ohio (D): This race has already taken a turn for the bizarre, with Democrats crying foul over a U.S. Chamber of Commerce ad that they say makes Sen. Sherrod Brown (D-Ohio) look bad. State Treasurer Josh Mandel (R) has raised lots of money, but we’re still waiting to see what the 34-year-old’s campaign is all about. And as we’ve noted before, Brown is formidable. (Previous ranking: 8)

7. Virginia (D): In a race that hasn’t changed much since it became clear that former governor Tim Kaine (D) and former senator George Allen (R) would run, the recent early debate between the two amounted to big news. Post-game handicapping suggested that Kaine had won the day, although it’s far from clear that a debate nearly one year before the 2012 election will have any significant impact on the dynamic of the race. We continue to believe that this race will be the marquee contest of the 2012 Senate cycle for three reasons: the size of the two personalities involved, the competitiveness of Virginia at the presidential level and the likelihood that this race will be very, very close. (Previous ranking: 7)

6. Montana (D): Democrats gleefully moved around a poll conducted by the Montana Chamber of Commerce that showed Sen. Jon Tester (D) leading Rep. Denny Rehberg (R) 42 percent to 37 percent. Other data suggests that the race is closer and, given the state’s Republican lean, it’s hard to imagine Tester winning by more than a point or two. But the very fact that Tester is in the game -- given the anti-incumbent sentiment in the country -- speaks to the fact that his personal brand may be able to weather the problems the national Democratic Party will have in the Last Best Place. (Previous ranking: 5)

5. Nevada (Republican-controlled): Normally, the fact that Sen. Dean Heller was appointed to the job -- on an interim basis -- that he is seeking to hold full-time next year would be a major political blessing. Not so much in a year like this one. Democrats note that Heller is the only person in Congress to vote for Rep. Paul Ryan’s budget plan twice -- once in the House, once in the Senate -- and Heller has been outraised by Democratic Rep. Shelley Berkley of late. The whole shebang in this race is Clark County (Las Vegas). Berkley needs to win it -- and win it big -- to offset losses in the more rural reaches of the state that Heller represented in the House. Lucky for her, Senate Majority Leader Harry Reid (D) showed her the way during his 2010 win. (Previous ranking: 4)

4. Missouri (D): Republicans continue to insist that Sen. Claire McCaskill (D-Mo.) is in deep trouble while Democrats insist that she is inoculating herself from President Obama and her party — look at her recent call for a permanent earmark ban — and that a lackluster field of GOP candidates could save her. Of the GOP field, National Republican Senatorial Committee Chairman John Cornyn (Texas) recently told the Wall Street Journal: “We have a three-way primary, and it really depends on who’s nominated, whether they are able to withstand what they know is coming at them in a general election.” (Previous ranking: 3)

3. Massachusetts (R): This race, more than any other, has been climbing up our list in recent months. That’s because a series of polls continue to show Democrats in better and better position. The most recent is a Boston Herald poll from last week that shows Elizabeth Warren taking a seven-point lead on Brown. More importantly, the poll showed Brown falling out of favor with Massachusetts voters, with his personal favorable and approval numbers dropping. And given the state’s heavy blue lean, he needs to remain a very popular Republican to win reelection. (Previous ranking: 6)

2. Nebraska (D): The question here is whether Sen. Ben Nelson (D) seeks reelection and that announcement is coming soon. If Nelson doesn’t run, this seat probably moves to the top spot on the Line with the GOP being heavily favored to pick it up. His exit may also open up the race to someone like Gov. Dave Heineman (R), who is so popular that he would be a shoo-in. As in Missouri, the GOP field here is something of a question mark. (Previous ranking: 2)

1. North Dakota (D): Democrats are convinced that the political handicapping world -- or at least the Fix -- has this race all wrong. Yes, they have a quality candidate and proven vote-getter in former attorney general Heidi Heitkamp. Yes, Rep. Rick Berg, the near-certain Republican nominee, is nowhere near as popular as Sen. John Hoeven was when he swept to the Senate in 2010. But North Dakota is likely to go strongly for the Republican presidential nominee -- no matter who it is -- in 2012, and it’s hard to imagine Heitkamp overperforming Obama by 10 or more points. (Previous ranking: 1)

Chris Cillizza contributed to this report