A fourth Democrat on the Senate Banking Committee plans to oppose Larry Summers' potential nomination to chair the Federal Reserve Board, something that could be devastating to his candidacy.

Sen. Jon Tester's (D-Mont.) office told Reuters that Tester plans to oppose Summers, who is widely considered the front-runner to be tapped by President Barack Obama in the coming weeks. His office didn't immediately respond to a request seeking confirmation.

President Obama is thought to prefer Summers over Fed Vice Chair Janet Yellen, who is the preferred candidate of many Democrats and many on the left.

The Senate Banking Committee must approve Summers' potential nomination before it goes to the Senate for full confirmation, something that looks increasingly unlikely. The Wall Street Journal reported last week that Democratic Sens. Elizabeth Warren (Mass.), Sherrod Brown (Ohio), and Jeff Merkley (Ore.) are expected to oppose Summers in committee.

Democrats hold a two-vote majority on the Banking Committee. If four Democrats vote against Summers, at least four Republicans will have to vote in favor for his nomination to get out of committee. That is unlikely, considering that no Republican on the committee besides Mike Johanns (R-Neb.) has expressed even cautious support of Summers.

Business Insider's Josh Barro reached out to other Democratic members of the Banking Committee last week, and besides Chairman Tim Johnson (S.D.), whom the Journal reports is supportive, only Sen. Chuck Schumer (N.Y.) was warm to the possibility of Summers as Fed Chair.

A breakdown:

Schumer thinks both Summers and Yellen would be "very good choices," according to spokesman Matt House.

Jack Reed (R.I.) "will consider any person the President chooses to nominate," per spokesman Chip Unruh.

Heidi Heitkamp (N.D.) "is closely examining all individuals who have been mentioned as potential candidates," according to a spokesperson.

Robert Menendez (N.J.): No comment.

Mark Warner (Va.): No comment.

Kay Hagan (N.C.) and Joe Manchin (W.Va.) did not respond to a request for comment.

In July, about one-third of Senate Democrats signed on to a letter circulated by Brown, urging Obama to appoint Yellen to the position.