Former congresswoman Stephanie Herseth Sandlin announced Monday that she will not run for South Dakota's open Senate seat in 2014, sharply increasing the likelihood that  Democrats will lose a seat their party now holds.

"While I know you share my confidence that working together we could win a statewide race next year, I’m also confident that the decision not to run is the right decision for Max, Zachary, me and our entire family," Herseth Sandlin wrote on her Facebook page.

Herseth Sandlin is apparently joined on the sidelines by U.S. attorney Brendan Johnson, the son of retiring Sen. Tim Johnson (D-S.D.), whose supporters last week indicated he would not run. Johnson has not officially announced his plans, though.

The early thinking had been that both Johnson and Herseth Sandlin might run for the seat, creating a potentially contentious primary.

Instead, former governor Mike Rounds, a Republican, is the big name in the race right now. Rick Weiland (D), a former congressional candidate and congressional aide, jumped into the race last week -- a move that was seen as an indicating that the younger Johnson wouldn't run.

Herseth Sandlin lost reelection to Rep. Kristi Noem (R) in 2010, in large part because of the national Republican wave that swept the country. Despite her loss, she remained personally popular in the Jackrabbit State through the end of her tenure, and Democrats believed she would have given them a good shot at holding Johnson's seat in a red state.

Herseth Sandlin is the granddaughter of two statewide South Dakota officeholders -- her grandfather, Ralph, was governor -- and the daughter of a longtime state legislator. She lost a congressional race against then-Gov. Bill Janklow (R) in 2002 but won a 2004 special election after Janklow was convicted of manslaughter following a car accident.