The Democratic Senatorial Campaign Committee is endorsing MJ Hegar in the crowded primary to challenge U.S. Sen. John Cornyn, R-Texas.

The move by the DSCC, the political arm of Senate Democrats, is one of the biggest developments yet in the nominating contest, which has drawn a dozen candidates — some more serious than others but no decisive frontrunners. The endorsement drew pushback from at least four of Hegar's competitors, two of whom accused national Democrats of snubbing more diverse candidates for Hegar, who is white.

Hegar, the former Air Force helicopter pilot and 2018 congressional candidate, entered the primary in April and has emerged as the top fundraiser. But polls show the race remains wide open as Democrats look to pick up where they left off from Beto O'Rourke's near-miss 2018 loss to the state's junior senator, Ted Cruz.

“Texas has emerged as a battleground opportunity for Democrats up and down the ballot, and MJ Hegar is the strongest candidate to flip the U.S. Senate seat," the DSCC's chairwoman, Nevada Sen. Catherine Cortez Masto, said in an announcement first shared with The Texas Tribune. "As a decorated combat veteran and working mother, MJ has both the courage and independence to put Texas first and is running on the issues that matter most to Texans: making health care and prescription drugs more affordable, protecting coverage for Texans with pre-existing conditions, and taking action to address climate change."

The backlash to the endorsement began with the campaign of Hegar rival Cristina Tzintzún Ramirez, which called the DSCC nod "tone-deaf to the the diverse Texas electorate." The campaign of another candidate of color, Dallas state Sen. Royce West, later denounced the endorsement as a "slap in the face" to black Texans.

In her own statement, Hegar touted that she has built a "Texas-sized grassroots campaign," one that has raised over $2 million and covered 11,000 miles across the state by her count. In order to make progress on the state's biggest issues, she added, "we need to defeat ... Cornyn and get more regular people like me representing us in Washington."

Hegar received some national support in the opening weeks of her campaign, including from VoteVets and End Citizens United, but major endorsements such as the DSCC's have been few and far between as the primary has proceeded. Hegar does have one tie to the DSCC: Her campaign manager, Preston Elliott, was the committee's deputy executive director during the 2016 cycle. Also, she met with Senate Minority Leader Chuck Schumer ahead of her campaign launch, as did at least one other candidate, West.

In any case, the DSCC endorsement is notable as some of the candidates have begun to sharpen their strategies to stand out with less than three months until the primary. Tzintzún Ramirez in particular has taken steps to more clearly position herself as the progressive alternative to Hegar, who has eschewed political labels while running a general election-focused campaign. Tzintzún Ramirez, a veteran organizer, was endorsed Friday by the labor-aligned Working Families Party.

"[The DSCC has] decided to ignore several more qualified and experienced candidates of color, who have done the work to transform the politics of our state, in favor of a former Republican," Tzintzún Ramirez campaign manager David Sanchez said in a statement. "It is disappointing that the national Democratic establishment is so afraid of progressive ideas that it will not even give Texas voters a chance to hear them."

Tzintzún Ramirez's campaign later launched a petition calling on the DSCC to retract its Hegar endorsement.

A second Hegar opponent, former Houston congressman Chris Bell, also referred to her as a "former Republican" while denouncing the DSCC's intervention as "another unfortunate attempt by Beltway politicians and insiders to tip the scales of democracy for what they think is best for Texas."

The label "former Republican" is an apparent reference, at least in part, to Hegar's participation in the 2016 GOP primary, which she has explained as a protest vote against Donald Trump. Hegar spokeswoman Amanda Sherman reiterated that in a statement Monday, saying Hegar "saw the threat that Donald Trump and his dangerous rhetoric posed for Texas and the security of our country." Sherman added that Hegar instead voted for Hewlett-Packard CEO Carly Fiorina, who had ended her campaign by that point.

Like Tzintzún Ramirez's campaign, West's did not hold back in criticizing the DSCC endorsement as an insult to candidates of color. West spokesman Vince Leibowitz said in a statement that the DSCC is "trying to lock African Americans out of the process" and called it a "very sad day for the Democratic Party."

The campaign of another African American contender, Houston City Councilwoman Amanda Edwards, additionally spoke out about the endorsement.

"We are quite confident Texas Democrats will decide for themselves who faces John Cornyn, irrespective of Washington insiders attempting to put a thumb on the scale," Edwards senior adviser Keir Murray said in a statement.

Republicans piled on as well — and like Tzintzún Ramirez and West, focused on the implications of the endorsement in the diverse primary field. State GOP Chairman James Dickey said in a statement that "Democrats talk about diversity in their party, yet this latest move proves they are all talk and no action." The National Republican Senatorial Committee issued a statement claiming national Democrats have "been forced to dismiss one of the most diverse fields in the country to do whatever they can to help Hollywood Hegar survive this brutal primary." And Cornyn's campaign sent out an email recapping the backlash, vowing that Cornyn will be ready for battle "no matter who limps out of the Democrat’s runoff."

Hegar's other primary competitors include Sema Hernandez, O'Rourke's 2018 primary opponent who got a stronger-than-expected 24% of the vote, and Michael Cooper, the runner-up in the 2018 primary for lieutenant governor.

The filing deadline was a week ago for the Senate primary and others in Texas. Despite lingering speculation since he ended his presidential campaign last month, O'Rourke reiterated in the days before the deadline that he would not run against Cornyn and the deadline passed without him filing.