Texas Rep. Steve StockmanSteve StockmanCruz will skip State of the Union Ethics: Lawmakers didn’t ‘knowingly’ break rules with Azerbaijan gifts Lawmakers deny knowledge of secret funding for 2013 trip MORE (R) is launching a primary challenge to Sen. John CornynJohn CornynPotential Cruz challenger: 'Don't close off your options' Report: Investor visa program mainly funds wealthy areas GOP senators avoid Trump questions on rigged election MORE (R), The Hill has confirmed with a Texas Republican Party spokesman.
Cornyn has long been thought vulnerable to a primary challenge, but until Stockman's entry into the race, he appeared to have escaped a prominent challenger.
“We are extremely disappointed in the way he treated his fellow congressmen and broke the 11th commandment and undermined [Sen.] Ted Cruz’s fight to stop Obamacare," Stockman said, citing former President Ronald Reagan's famous guideline that Republicans shouldn't speak ill of other Republicans.
Cornyn drew the ire of conservatives after withdrawing his support for, and becoming an outspoken critic of, Cruz's strategy to shut down the government in order to defund ObamaCare.
Texas Republican Party spokesman Spencer Yeldell confirmed that Stockman has withdrawn his application for his current congressional seat and filed for the Senate race. He said the state party would remain neutral in the primary, as it does in all races.
Cruz, notably, declined to endorse Cornyn's reelection bid, saying he plans to stay out of primary fights that endanger incumbents. His office reiterated that he wouldn't get involved in the race Monday night.
At least one prominent national conservative group is taking a look at the race.
"We haven't decided yet whether we will endorse Steve Stockman, but we're excited about the potential here. Texas deserves two conservative fighters in the Senate, not just one," said Matt Hoskins of the Senate Conservatives Fund, which endorsed Cruz in his race.
Hoskins said of Cornyn: "He's part of the problem in Washington, and voters deserve an alternative."
But Stockman will face a difficult fight. He had only $32,000 in the bank at the end of September, a paltry sum for a campaign in Texas, where media prices, combined with the costs of traveling the state push campaign spending into the tens of millions.
And he'll have to answer questions during the primary about his apparent failure to disclose various business dealings in his congressional financial disclosure forms, as revealed in a recent Houston Chronicle investigation.
His penchant for off-beat comments and crusades — he's an outspoken advocate for impeaching President Obama, and his campaign is selling a bumper sticker that reads, "If babies had guns they wouldn't be aborted" — might make him an easy target for establishment attacks.
A survey from the Democratic firm Public Policy Polling out in November showed Cornyn at 46 percent approval among GOP primary voters, and 49 percent said they'd like a more conservative candidate in the race.
But Cornyn has been preparing for reelection with the possibility of a primary fight in mind, raising nearly $7 million by the end of September and launching an early campaign ad touting his conservative bona fides.
For his part, Stockman said he's looking forward to a "vigorous campaign," and though he sounded a cautious note on his run, he drew an implicit comparison to Cruz's upset in an appraisal of his own chances.
“I don’t know that I can beat him, but I am sure going to try,” he said. He added that he thinks he has a chance in the race because, “In Texas, conservative policies win over stabbing fellow Republican in the back.”
He's also been a fierce defender of Cruz's controversial push to defund ObamaCare.
"I always thought Cruz’s ploy, which was ridiculed by many of the party members, was brilliant," he said in late October. "People now know he’s a man of principle. So, I think it was smart."
In response to Stockman's announcement, Cornyn's campaign manager Brendan Steinhauser touted the senator's conservative support and vote ranking from National Journal.
“Endorsed by Texas Right to Life and ranked as the 2nd most conservative Senator in America, Sen. Cornyn looks forward to discussing his conservative record with Texans," he said.
And the National Republican Senatorial Committee pledged to support Cornyn, a former NRSC chairman, in what spokesman Brad Dayspring called a "head-scratcher" of a primary challenge.
"John Cornyn is one of the most conservative members in the Senate and strong leader for the state of Texas. We are proud to support Senator Cornyn, and while this primary challenge is quite the head scratcher, it will be defeated," he said.
— This post was updated at 9:00 p.m.