Democratic candidates for the U.S. Senate are now explicitly campaigning against one of their potential colleagues, Sen. Kyrsten Sinema (D-Ariz.) — branded by one as a "sellout" for opposing filibuster changes to enact party priorities.

Why it matters: It's an evolution of an increasingly popular strategy among Democrats: turning legislative inaction to their advantage by casting themselves as the "50th vote" for programs or the filibuster changes needed to pass President Biden's agenda.

• Former Rep. Abby Finkenauer, a Democrat running to challenge Sen. Chuck Grassley (R-Iowa) in November, called Sinema a "sellout" in a video posted to Twitter on Thursday.
• "[Sinema] is there for what reason I don't know, but refuses to actually do anything that works for working families," said Finkenauer. The former frontline House member styles herself as a moderate.
• John Fetterman, the progressive Pennsylvania lieutenant governor running for Senate, told Axios in a statement: "Now more than ever, Democrats need to vote like Democrats. Protecting the right to vote > Filibuster.”
• Sinema’s office did not immediately respond to a request for comment.

The backdrop: Sen. Joe Manchin (D-W.Va.) has long been a punching bag for progressive candidates. Like Sinema, he's opposed to altering the filibuster without bipartisan support.

• “Nearly every party-line response to the problems we face in this body — every partisan action taken to protect a cherished value — has led us to more division, not less,” Sinema said in a speech just before President Biden arrived on Capitol Hill to lobby his fellow Democrats.
• The stance earned her praise from the chamber's top Republican, Senate Minority Leader Mitch McConnell. He told reporters: “It was extraordinarily important and she has, as a conspicuous act of political courage, saved the Senate as an institution.”
• The trend of candidates contrasting themselves not just with Manchin but another of the Senate's most conservative Democrats seems to be gaining steam beyond progressives.

Driving the news: Finkenauer urged voters to "expand the Democratic majority" in her video. In a follow-up tweet, she vowed to "vote for filibuster reform to pass voting rights legislation."

• On Tuesday, Rep. Val Demings (D-Fla.), who's running for the Senate, signaled her support after Biden said he backed changes to the filibuster.
• "I'll be the 50th vote Democrats need to end the filibuster and pass voting rights protections," she said in a campaign email."
• This is a shift from just a few weeks ago, when Axios' Sophia Cai reported on the spate of Dem candidates for Senate campaigning on the procedure (filibuster reform) but not the person (Sinema, Manchin).

Don't forget: Democratic pressure against Sinema will only get louder this weekend.

• Relatives of the late Martin Luther King Jr., including his namesake Martin Luther King III and his wife, Arndrea Waters King, will hold a rally in Phoenix to urge the senator to change her position.
• They'll be joined by the Rev. Al Sharpton and other civil rights leaders.
• The president hosted her and Sen. Joe Manchin (D-W.Va.) at the White House on Thursday evening to continue his own lobbying.