Maine Democratic Rep. Chellie Pingree has opted to run for reelection rather than challenge former Independent governor Angus King for the state’s open Senate seat.

“There is much at stake in this election, and although the prospect of running for and possibly serving in the United States Senate was very exciting, in the end I concluded that I will best serve the people of Maine by running for reelection to the House,” Pingree said in a statement.

The Plum Line’s Greg Sargent broke the news of Pingree’s decision.

Pingree was considered the early Democratic favorite for the seat, but when King, a close friend, opted to run, Pingree’s calculus changed.

The question now is whether Democrats will field a strong candidate for the seat or effectively line up behind King, a former Democrat who may caucus with them in the Senate.

But it remains to be seen whether the left would find King acceptable. Pingree is seen as the more outspoken champion of liberal causes.

Some liberals are already speaking out.

“Angus King was willing to create a three-way race that handed this Senate seat to a far-right Republican — a precise repeat of what happened in Maine’s 2010 three-way gubernatorial rac,” said Adam Green, co-founder of the Progressive Change Campaign Committee, a group that tried to draft Pingree into the Senate race. ”Chellie Pingree’s decision was based on what is best for the people of Maine and the future of progressive causes, not based on what was best for herself, and we look forward to continuing our work with her around progressive fights in Congress.”

Pingree’s House colleague, Rep. Mike Michaud (D), also opted not to run for Senate. Former governor John Baldacci (D), among others, is still considering his options.

On the GOP side, state Attorney General Bill Schneider is the early establishment favorite.