Already facing big disadvantages in Senate campaigns this year, Democrats got another dose of bad news today as former Sen. Bob Kerrey announced he won't run in Nebraska.

Kerrey offered Democrats a glimmer of hope in their toughest state. When Sen. Ben Nelson, D-Neb., announced his retirement in December, the state immediately topped the list of open, competitive seats Democrats will have to defend in 2012. Without Kerrey, who served in the U.S. Senate from 1989 until 2001, Democrats will have a much tougher time defeating the winner of the three-way Republican primary in this GOP-leaning state.

But Democratic party officials are putting on their game face regarding the challenge.

"As we have seen in the last several weeks, Republicans are at each other's throats in Nebraska.  The Republican primary in the state has become a proxy war between Mitch McConnell's ethically challenged candidate Jon Bruning and Jim DeMint's tea partier Don Stenberg, which will provide an opportunity for Democrats to remain competitive," Democratic Senatorial Campaign Committee spokesman Matt Canter said of Kerrey's decision.

"We continue to play offense this election cycle in Massachusetts, Nevada, Arizona, and Indiana, and remain fully confident that we will hold the majority next year."

The slate of Senate elections will heavily favor Republicans in 2012. Democrats will defend 23 seats, while Republicans will defend 10. Six Democratic incumbents have declined to run: Sens. Daniel Akaka  of Hawaii, Jeff Bingaman of New Mexico, Kent Conrad of North Dakota, Herb Kohl of Wisconsin, Nelson, and Jim Webb of Virginia. Republicans, by contrast, have only one vulnerable seat left open, vacated by retiring Arizona Sen. Jon Kyl.

Democrats will face between eight and 11 competitive races to defend seats they currently hold. The most competitive Democratic-held seats represent Hawaii, Florida, Michigan, Missouri, Montana, New Mexico, North Dakota, Ohio, Virginia, and Wisconsin.

Republicans, meanwhile, will face only two to four competitive races to hold onto seats. Competitive GOP-held seats will be contested in Arizona, Indiana, Massachusetts, and Nevada.

Democrats currently hold a Senate majority of 53 seats to Republicans' 47, counting the two independent senators who caucus with Democrats, Bernie Sanders of Vermont and Joe Lieberman of Connecticut.  If Republicans can defend their seats and win in four states where Democrats are vulnerable, the GOP will retake the Senate majority.

The most prominent Senate races of 2012 figure to take place in Massachusetts and Virginia.

In Massachusetts, Republican Sen. Scott Brown will face his first reelection challenge since he stunned Democrats in January 2010 by defeating Martha Coakley in the special election to replace the late Sen. Ted Kennedy, eliminating Democrats' 60-seat supermajority in the upper chamber. Elizabeth Warren, the consumer advocate who designed President Obama's consumer financial protection board, figures to run a well-funded campaign against him focusing on income inequality and the struggling middle class.

In Virginia, former governor and Democratic National Committee chairman Tim Kaine is running against former Sen. George Allen, who lost in 2006 to Webb amid a disastrous campaign that included Allen's infamous utterance of the confusing, apparent racial epithet "macaca, " which Allen later said had nothing to do with race. Both are considered strong candidates, and, like in many states, the contest may come down to President Obama's performance in the state.