Sen. Elizabeth Warren (D-Mass.) is putting her liberal star power behind two likely Democratic nominees for U.S. Senate: Rep. Bruce Braley (D-Iowa) and South Dakota Democrat Rick Weiland.

"Bruce Braley led the Populist Caucus in the House of Representatives – and is focused on economic fairness, investing in education, and addressing our retirement crisis," Warren wrote in a fundraising e-mail sent out Thursday in conjunction with the Progressive Change Campaign Committee. "Rick led South Dakota’s AARP and federal emergency management in his state, and he worked as a top Senate aide."

Braley was tripped up earlier this week when video surfaced of him referring to popular Sen. Charles E. Grassley (R-Iowa) as a "farmer from Iowa who never went to law school." Braley later apologized for the remark, which he made at a January fundraiser in the context of a discussion about Grassley possibly becoming the next chairman of the Senate Judiciary Committee.

Polls show Braley leading his Republican opponents. But Weiland, a former Tom Daschle aide, is an underdog. He's likely to face former governor Mike Rounds (R) in the general election.