Liberal Democrats seething about Arizona Sen. Kyrsten Sinema’s moderate stances got a challenger to take her on Monday, but Rep. Ruben Gallego’s announcement also creates the potential for a messy three-way contest in a battleground state. 

The campaign arm for Senate Democrats, whose 51-seat majority control in this Congress includes Sinema even though she officially became an independent after the November election, wouldn’t comment on Gallego’s candidacy Monday.

But Sinema’s fellow moderate Democrat, Sen. Joe Manchin III of West Virginia, said he’d back Sinema should she decide to run, and he urged Senate Democratic leaders to do the same. 

“I’m totally supportive of Kyrsten Sinema,” Manchin told reporters Monday. “She’s earned the support” by being “a centrist willing to take tough votes and do the right thing.”

Leadership “should support someone who brings some peace, if you will, or some rational thinking on some of this stuff without being pushed far left or far right.” Manchin, whose seat is also up in 2024 and who has not said whether he will run again, called Gallego a “a good person” but said he had a “good relationship with Kyrsten. I trust her I understand where she’s coming from, and she’s always going to try to find the middle.”

Sinema, who served three terms in the House before beating Republican Martha McSally in 2018, has not publicly said whether she will run for reelection next year. Last month, she formalized her estrangement with the Democratic Party and registered as an independent.

Gallego, a five-term House member from Phoenix, isn’t waiting for her decision. 

“I have been deeply humbled by the encouragement I have received from the people of Arizona, and today I am answering the call to serve,”  Gallego said in a campaign announcement. “I’m running for the U.S. Senate because the rich and the powerful don’t need any more advocates in Washington — but families who can’t afford groceries do. As a Marine, I never back down from a fight, and in the Senate I’ll fight to make sure every Arizonan has the same chance that I had at the American Dream.”

“The problem isn’t that Senator Sinema abandoned the Democratic Party — it’s that she’s abandoned Arizona,” Gallego said, adding that she had sided with the pharmaceutical industry and Wall Street “at our expense.”

Sinema had already been facing a movement to challenge her in the Democratic primary, an effort that gained steam when she sided with Republicans against relaxing Senate rules that require 60 votes to end a filibuster on most legislation.

“We are thrilled that there’s now a Democratic candidate in this race ready to take on Kyrsten Sinema and win,’’ said Sacha Haworth, spokeswoman for the Replace Sinema campaign. “As she jet sets with the international elite and does favors for her Wall Street donors at the expense of working Arizona taxpayers, Kyrsten Sinema shows us daily that she is only out for herself, and it’s time for new leadership.”

While Sinema has tangled with the party’s leadership on a number of issues, she was also a steadfast supporter of key parts of the Biden agenda, including gun control legislation, efforts to protect same-sex marriage and major spending packages. Nora Keefe, spokeswoman for the Democratic Senatorial Campaign Committee, declined to comment.

“Republicans have suffered resounding Senate defeats in Arizona the last three election cycles in a row, and we are confident we will stop Republicans in their effort to take this Senate seat,’’ Keefe said.

But Republicans expressed hope that Gallego and Sinema would split the Democratic vote and the GOP would reverse the party’s Arizona Senate losing streak that dates back to 2018.

“The Democrat civil war is on in Arizona. [Senate Majority Leader] Chuck Schumer has a choice: stand with open borders radical Ruben Gallego or back his incumbent, Sen. Kyrsten Sinema,” said Philip Letsou, a spokesman for the National Republican Senatorial Committee.

The Gallego campaign said it has assembled a team of veteran Democratic operatives who helped Sens. John Fetterman of Pennsylvania, Raphael Warnock of Georgia and Mark Kelly of Arizona win.

Gallego, 43, is a Harvard graduate and former Marine who served in Iraq and was first elected to Congress in 2014. A member of the Congressional Progressive Caucus, he has backed a number of liberal proposals, including a $15 hourly minimum wage and marijuana legalization. He also chairs the Congressional Hispanic Caucus’ political arm, BOLD PAC.

Gallego, who would be the first Latino senator from Arizona, released a campaign video in English and Spanish that traces his biography as the son of a struggling single mother who immigrated to the U.S. from Colombia. He recalled sleeping on the floor of the family’s apartment and hearing his mother sob every night as she worried about paying the bills on her secretary’s salary.

“Statistically, I was never supposed to end up even in college,” he said. He credits his success to his family, who didn’t laugh when he announced his intention to attend Harvard, and the teachers who supported him.

Last week, Rep. Greg Stanton, Gallego’s Democratic colleague from Phoenix, announced that he was forgoing a Senate campaign. Stanton, the former mayor of Phoenix, said it wasn’t the right time for him to run.

Arizona is a politically purple state with a strong independent streak. Democrats scored two major midterm victories: The party’s gubernatorial nominee, Katie Hobbs, defeated Republican Kari Lake, and Kelly beat Republican Blake Masters by almost 5 percentage points. Kelly’s win, in particular, was largely due to his gains among Latino voters, who make up about 20 percent of the state’s voting population, according to an analysis of the 2022 results by FiveThirtyEight.

Both Lake and Masters are potential 2024 Senate candidates.

Minutes after Gallego officially launched his campaign, the NRSC issued a press release painting him as a “radical” who favors open borders. Gallego had about $1.1 million in his campaign account as of Nov. 28, according to a filing with the Federal Election Commission.