Major outside support is arriving for MJ Hegar as she works to advance in the crowded Democratic primary to challenge U.S. Sen. John Cornyn, R-Texas.

VoteVets, the group that endorsed Hegar shortly after she launched her campaign last year, is launching a $3.3 million TV ad buy boosting her a month before the primary, according to details first shared with The Texas Tribune. The buy starts Tuesday and will run for two weeks on broadcast, cable and satellite.

The buy includes a 30-second spot that highlights how Hegar, a former Air Force helicopter pilot, has "taken on the toughest fights," mentioning her heroism in Afghanistan and then the "fight of [her] life": the birth of her son Daniel, which required two blood transfusions. The experiences, a narrator says, show why Hegar "will fight for quality affordable health care and protect coverage for people with preexisting conditions."

"MJ Hegar: Texas tough," the narrator concludes.

The buy is a big development in the 12-way primary, which has been full of uncertainty about which candidates will be able to make it to a widely expected runoff. Hegar has been the top fundraiser and got the endorsement of the Democratic Senatorial Campaign Committee late last year, but polls show large portions of voters are still undecided and no candidate has broken away.

The buy marks the first significant outside spending in the primary, and its $3.3 million price tag is even more significant. That is more than the entire Democratic field spent in the fourth quarter.

The buy is being made through VoteVets' super PAC, likely a sensitive subject in the primary as Democrats increasingly rail against big money in politics. While Hegar has not explicitly sworn off super PAC support, she criticized a political action committee supporting one of her opponents, Cristina Tzintzún Ramirez, after it came to the latter's aid last month on Twitter after Hegar's criticism of a fundraising email that Tzintzún Ramirez sent out. Along with VoteVets, one of Hegar's early national endorsers was End Citizens United, which works to elect Democrats committed to campaign finance reform.

Cornyn is awaiting the eventual Democratic nominee with a $12.1 million war chest.