Senate Democrats knew that 2012 wasn’t going to be a picnic. Republicans need to gain only four of 23 Democrat-held seats up for election next year to take control of the upper chamber. And President Obama’s low approval ratings in swings states always meant a tough test.

But the Democratic Party had been doing a good job of limiting the size of the playing field. No longer. Facing the prospect of a bruising presidential-year race, six Democratic incumbents have so far opted to retire rather than run again.

With the announcement this week that four-term incumbent Sen. Herb Kohl of Wisconsin was walking away, there are now four vacant seats in swing or red states: Wisconsin, Virginia, North Dakota and New Mexico. That’s means millions more dollars that Democrats must pour into defending their turf.

The other two retirements, Sen. Daniel Akaka in Hawaii and Sen. Joe Lieberman in Connecticut, are unlikely to produce much trouble for the party. Like the two Republicans retiring, Sen. Kay Bailey Hutchison of Texas and Sen. Jon Kyl of Arizona, Akaka and Lieberman leave Democrats with easy territory to defend.

When the cycle started, Democrats were looking at defending nine seats in their party’s control. Now, the map has grown to at least 13. That list will shrink or grow depending on how Obama fares. Sen. Bob Casey of Pennsylvania and Sen. Bill Nelson of Florida, for example, go from likely winners to likely losers if Obama and the economy are still in the doldrums next year.

Democrats are looking at a baker’s dozen of tough races and 10 safe seats while Republicans, by chance, have a total of 12 seats up for election this year and only two incumbents – Sen. Scott Brown of Massachusetts and Sen. Dean Heller of Nevada – are in any perceivable danger. Brown is in much stronger position than expected and Democrats have struggled to find a top-tier challenger. Heller is brand new to the Senate and will have little advantage of incumbency, but to Republicans, anyone other than the scandal-soaked former occupant of the office, John Ensign, looks like a four aces.

As Democrats begin the process of triage – deciding which seats can’t be saved in order to preserve resources for expensive races in competitive states – they can at least take heart in knowledge that their current problems are a result of past success. If Democrats hadn’t been so successful in 2006 by running against George W. Bush and the Iraq war, they wouldn’t be spread so thin this year.

Now, on to the Power Politics Senate Democrats misery index. Here are, ranked by degree of difficulty, the eight toughest races for Democrats next year:

1) Montana – It’s already getting ugly in Montana. Sen. Jon Tester and state Republicans had their first dustup when the senator got into a scrape with a GOP campaign video stalker in a Missoula parking lot. The videographer says Tester tried to run him over after the campaign operative stuck his camera in the senator’s face. The Tester team says the he was just backing up, albeit with vigor. Tester won his 2006 race amid a Democratic wave and against a weakened incumbent in Sen. Conrad Burns – and it was still a narrow win. Now, with Montana back to its bright-red roots, Tester is squirming over his votes for President Obama’s national health care law and global-warming fees. Rep. Denny Rehberg, who already represents the entire state, has a clear path to the Republican nomination and strong fundraising potential.

2) North Dakota – Freshman Rep. Rick Berg is taking the plunge into a Senate run after less than five months in office. That may be a little cheeky by most political standards, but the retirement of longtime Democratic Sen. Kent Conrad is something of a Republican bonanza. The state has already been trending Republican and Democrats are still without a big-name candidate.

3) Missouri – Missouri Rep. Todd Akin, a conservative five-term incumbent from suburban St. Louis, has entered the Republican field looking to unseat Sen. Claire McCaskill, joining former state Treasurer Sarah Steelman. While Republicans aren’t enthused at the prospect of what could be a very rough primary, they certainly like their chances against McCaskill. Aside from the scandals surrounding a plane owned by the freshman Senator and her developer husband, MCaskill is mostly bogged down by the strong anti-Obama sentiment in Missouri. She was one of his primary spokeswomen during the 2008 campaign and will struggle to distance herself from the president.

4) Wisconsin – Get out your bullhorns, folks. Wisconsin may be the battleground of all battlegrounds in 2012. It’s a must-win state for Obama and now will have a vacant Senate seat with Kohl’s retirement. Former governor and Health and Human Services Secretary Tommy Thompson looks like a shoo-in for the Republican nomination. On the Democratic side, former Sen. Russ Feingold, who was knocked out of office by Sen. Ron Johnson as the state bolted to the right in 2010, would be the automatic frontrunner if he decides to try again. But as the bitter battle over government worker unions this year showed, whoever runs in this polarized state will be in for a brawl.

5) Virginia – Virginia will also have a two-front war in 2012 as Obama tries to become the first Democratic presidential candidate since Franklin Roosevelt to win the commonwealth twice and the parties clash over a Senate seat left vacant by the retirement of freshman Sen. Jim Webb. Former Gov. Tim Kaine has left his spot as Democratic National Committee chairman to run for the seat while Republicans will choose between the man Webb beat in 2006, former Sen. George Allen, and Tea Party-favorite Jamie Radtke. This race will come down to Obama’s popularity in the state. Kaine has bet his career on Obama. It will pay off if higher black turnout and a strong showing in the government-dependent northern part of the state give Democrats an electoral edge. But if Virginia voters are still feeling feisty as they were in 2009 and 2010, Kaine will come up short.

6) Nebraska – Health care remains a huge problem for Democratic Sen. Ben Nelson. Nelson’s initial support for Obama’s national health care law destroyed the aura of invincibility around the second-term incumbent and former governor. Obama and his law remain direly unpopular in the state and despite many efforts by Nelson to stand apart, he hasn’t yet gotten out of Obama’s shadow. Republican state Attorney General Jon Bruning looks to be on his way to a decisive primary win with both establishment and Tea Party support.

7) New Mexico – Former Republican Rep. Heather Wilson is cruising to her party’s nomination for the seat being vacated by the retirement of Democrat Jeff Bingaman. Democrats meanwhile have coalesced behind second-term Rep. Martin Heinrich, who won Wilson’s former seat when she left the House for an unsuccessful 2008 Senate bid. There may be some primary tussles ahead on both sides, but for now this looks like a contest between two moderates from the middle of the state. Republicans are benefitting from the popularity of new Gov. Susana Martinez while Democrats are hoping that a strong showing for Obama among Hispanics – now 42 percent of the state’s population – will carry the day.

8) Ohio – Republicans are increasingly eyeing freshman Sen. Sherrod Brown for a 2012 takedown. Brown, like his fellow freshman Tester in Montana, is a liberal in a conservative state. He defeated incumbent Sen. Mike DeWine amid a public repudiation of the Iraq War and a badly faltering state Republican Party. But the party is mended, Iraq is over and Barack Obama is none too popular with Buckeye State voters. Republicans are looking for potential contenders, and with 13 of the state’s 18 House members now Republicans there are lots of potential candidates. One to watch is Rep. Jim Jordan, head of the conservative Republican Study Committee. If Brown is looking for a role model for dealing with low presidential approval ratings he need only cast his eyes across the Ohio River to fellow freshman Sen. Joe Manchin of West Virginia, who is making a career out of fighting with the White House. Those scraps took Manchin out of the Power Play endangered eight for the first time this edition.

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