Former Sen. Russ Feingold (D-Wis.) officially has endorsed Rep. Tammy Baldwin (D-Wis.) for her Senate race, the latest sign that Baldwin has united Wisconsin’s Democrats and will be a top recipient of liberals’ donations in coming months.

Today, I’m proud to throw my support to a candidate who will keep the fight for progress, the middle class and equality alive in the U.S. Senate: our good friend, Tammy Baldwin, Feingold, a liberal icon, said in an email to supporters before asking for donations to Baldwin’s campaign.

Baldwin is running for the seat of retiring Sen. Herb Kohl (D-Wis.), and it appears as though she will have a clear shot at the nomination — and will be able to bank lots of campaign cash while the Republicans duke it out.

A potentially nasty primary is brewing on the Republican side, with former Gov. Tommy Thompson (R) already taking harsh criticism from Washington-based conservative groups who are backing former Rep. Mark Neumann (R-Wis.) in the race. State Assembly Speaker Jeff Fitzgerald is also in the GOP primary.

Baldwin would be the first openly gay senator in U.S. history, and is known as a liberal populist who has focused her campaign on economic issues. She has been embraced by liberal groups, and alongside Elizabeth Warren, who is running for the Senate in Massachusetts and is sharing a fundraising PAC with Baldwin, she seems to be liberals’ favorite for the election cycle.