Former Mississippi Democratic Rep. Travis Childers announced Friday he's running for Senate, giving Democrats a top-tier candidate if Sen. Thad CochranWilliam (Thad) Thad CochranWhite House requests B for disaster relief GOP establishment doubts Bannon’s primary powers Whatever you think the Alabama special election means, you’re probably wrong MORE (R-Miss.) loses his primary.

Childers's decision just before the Saturday filing deadline indicates Democrats believe Cochran is highly vulnerable to state Sen. Chris McDaniel in the June primary. If the longtime senator goes down, Democrats hope they could pull off the upset in the deeply red state; eyeing missteps or gaffes by McDaniel could turn into a repeat of other unlikely contests they pulled off against weak GOP nominees. 

A former Blue Dog congressman, Childers said in a statement he’s running “to make sure that all Mississippians have a Senator in Washington looking out for them.”

“What I know is that the old ways of Washington aren’t working, and a new breed of partisanship isn’t the answer,” he said.

Childers added, in his campaign he’ll push for a balanced budget amendment, a pet issue for Republicans but one that Democrats usually avoid. First elected in a surprise special election in 2008 to replace then-Rep. Roger Wicker (R) when he moved to the Senate, he won a full term later that year. 

The conservative Democrat voted against the Affordable Care Act in 2010, but it wasn't enough to spare him from a loss in the GOP wave that November. 

Cochran is considered one of the most vulnerable Republicans facing a primary challenge in the nation, and McDaniel has the backing of nearly every national conservative group in his bid to unseat the incumbent.  

Democrats believe they’d have a shot at McDaniel because of his penchant for off-color comments similar to those that toppled other Republican candidates in winnable races in recent years. And in Childers, they have perhaps the only possible Democrat who could win statewide.

A similar scenario played out last year in Indiana. When longtime Sen. Richard Lugar (R-Ind.) lost to state Treasurer Richard Mourdock, then-Rep. Joe Donnelly (D-Ind.) had an uphill climb. But once Mourdock made controversial comments in debate on rape and abortion, Donnelly, also a Blue Dog, pulled off the unlikely win. 

Still, Mississippi will be a heavy lift even if Republicans enter the general election with a weak candidate. Obama won only 44 percent of the vote in 2008, and Democratic turnout typically falls off in a midterm year.

The state has a long history of Democratic representation in the Senate, but hasn't elected a Democratic Senator since the 1980s, and last contested a seat in 2008 — when, in a watershed Democratic year, former Democratic Gov. Ronnie Musgrove lost to Sen. Roger Wicker by 16 points.

But with Republicans adding Colorado to their list of competitive races after nabbing a strong recruit there this week, Childers' candidacy offers Democrats a much-needed offensive opportunity as they work to hold onto their majority this cycle.

—This piece was updated at 4 p.m.