One of the reasons Senate Democrats are so bashful about changing the rules to weaken the power of the minority is that most folks expect that’s exactly where Democrats will be after next year.

Republicans now have 47 seats in the Senate, meaning they need to gain only four seats to take control. Chance and a successful Democratic cycle in 2006 means that 23 seats controlled by Democrats will be up in 2012, compared to just 10 for Republicans.

The number that matters, though, is how many races will be competitive. And that’s where the Democrats have a real problem.

Republicans are only likely to be playing defense on two of the 10 seats. Sen. Scott Brown, R-Mass., will be in the fight of his life with his state’s Democratic voters heading to the polls for a presidential election. In Nevada, scandal-soaked Sen. John Ensign will have to fight to hold on to his swing-state seat.

We can’t yet know what primary upsets may be in store for Republicans. The Tea Party Express has already started calling out incumbents, starting with Maine’s Olympia Snowe. The group’s candidates lost winnable seats for Republicans in Delaware and Nevada in 2010, so the list of potential Republican defeats could certainly grow.

But it doesn’t seem likely that Republicans could sabotage enough victories to even get close to the number of states Democrats will have to defend.

While President Obama’s success or lack of it in the next year will do a lot to change the shading of the political map, Democrats will have to be prepared to defend at least 13 seats. Some of that baker’s dozen, like the races in Wisconsin, New Jersey, Connecticut, Pennsylvania and Michigan, are tough climbs for Republicans unless there is just the right combination of candidates and national climate. But the remainder promise to be brutally hard for the Democrats to defend.

Here, from hardest to hold to easiest to defend, are the eight biggest Democratic Senate trouble spots for 2012.

1) Montana – In 2006, Republican Sen. Conrad Burns ran a weak reelection campaign in a difficult year, which allowed Jon Tester, then a state Senator, to edge him out in a narrow race. Now, Tester is saddled with votes in favor of President Obama’s national health-care law and support for global-warming fees. The state’s at-large Republican congressman, Denny Rehberg, has already declared his candidacy. Tester has sought to distance himself from unpopular provisions of the health law, but the odds are stacked against him.

2) North Dakota – Democratic Sen. Kent Conrad’s decision to not seek reelection would seem to all but guarantee a Republican victory here. Jus last year, the state boasted an all-Democratic congressional delegation and Barack Obama had scored 44 percent of the vote in 2008, the best showing for a Democrat since 1976. Now the delegation is two-thirds Republican and Obama may not even get 30 percent of the presidential vote next year.

3) West Virginia – There may be no state in America where President Obama is a greater political liability than West Virginia. Not only is the state socially conservative, but Obama’s crackdown on the coal industry has made this longtime Democratic stronghold into Republican-friendly territory. Sen. Joe Manchin won the chance to serve out the late Robert Byrd’s Senate term in 2010, but the popular governor had to pull out all the stops to secure a victory. With Obama at the top of the ticket and Republicans making sure to point out every Manchin vote for the president’s agenda, Manchin’s stay in Washington could be a brief one.

4) Virginia – The decision by Sen. Jim Webb to not seek a second term was a sign of the times in Virginia. Webb narrowly unseated incumbent Sen. George Allen in 2006 largely on the basis of Webb’s opposition to the Iraq war. Now, Virginians seem satisfied with the leadership of conservative Republican Gov. Bob McDonnell and just swept three Democratic House incumbents from office in 2010. Republicans already have two strong contenders in Allen, looking to return to the Senate, and Tea Party star Jamie Radtke. Democratic chances will depend on Obama’s popularity in the Old Dominion. The best candidate for the party is former Gov. Tim Kaine, who now runs the Democratic National Committee for Obama, so his fortunes would rise and fall on Obama’s own efforts at political revival.

5) Missouri – Sen. Claire McCaskill has been working overtime to show her bipartisan tendencies and downplay her past support for President Obama’s national health-care law, and with good reason. Last year, Missouri voters overwhelmingly passed a constitutional amendment barring the law from taking effect in the state and Republicans picked up a House and Senate seat with campaigns that fiercely targeted the law. McCaskill was an early supporter of Obama, who made McCaskill a primary surrogate in his battle to woo female voters during his bitter primary election fight with Hillary Clinton. Republican former state treasurer Sarah Steelman is already in the race and will be a prodigious fundraiser.

6) Nebraska – Health care is the big problem for Sen. Ben Nelson, too. Just two years ago, it seemed likely that the moderate, popular former governor would have the seat for as long as he wanted. But, after Nelson’s vote for President Obama’s national health-care law, polls suddenly showed his approval ratings tanking. Nelson ultimately voted against the final version of the bill, but his initial support for the legislation had already done its damage. Republican state Attorney General Jon Bruning has already started his campaign against Nelson, whose name will be just below Obama’s on Nebraska ballots.

7) Ohio – First-term Democratic Sen. Sherrod Brown was one of the great Democratic success stories of 2006. Brown unseated incumbent Sen. Mike DeWine as part of a Democratic sweep through the Buckeye State. While other Democratic swing-state successes that year came from moderates like Tester and Webb, Brown managed his victory as a liberal. Ohio went back to its Republican roots in a big way in 2010, swinging five House seats from Democratic to Republican control. Ohio voters also gave freshman Sen. Rob Portman a landslide victory and chose Republican Gov. John Kasich. Brown’s support for President Obama’s health-care and environmental initiatives makes him vulnerable to the right Republican candidate. Possible Republican challengers could include longtime Rep. Jim Jordan and a potential rematch with DeWine, just elected state attorney general in 2010.

8) Florida – Democratic Sen. Bill Nelson’s bid for a third term comes at a difficult time. The Sunshine State took a sharp right turn in 2010, electing staunch conservatives to the governor’s mansion and Senate and flipping four House seats from blue to red. President Obama will push hard to keep Florida in play in 2012, but the terrain will still be hospitable to the right Republican candidate. The dream candidate for the state GOP is former Gov. Jeb Bush, who remains hugely popular there, even four years after leaving office. National Republicans have an interest in seeing Bush run, too. It would not only set him up for a future presidential contest, but a successful run by Bush would help next year’s Republican presidential nominee carry the state.

Chris Stirewalt is FOX News’ digital politics editor. His political note, Power Play, is available every weekday morning at FOXNEWS.COM.