Senate Minority Whip John Cornyn will face Rep. Steve Stockman in the GOP primary for Senate next year in Texas.

In a surprising move on the last day for candidates to file for federal office in the Lone Star State, Stockman promised “a vigorous campaign” against Cornyn in an interview with The World Net Daily.

Stockman, who was elected to the House in 2012 after serving a single term in the 1990s, said his impetus for running was Cornyn’s decision to not sign the letter with Sen. Ted Cruz that eventually instigated the government shutdown. He had not informed Cruz of his decision to run, according to the interview.

Monday was the filing deadline for Texas federal candidates, and Cornyn wasn’t expected to receive a serious challenge. However, the Republican Party of Texas confirmed that Stockman’s paperwork was turned into the committee. In Texas, state parties approve candidate applications before they are processed by the Texas secretary of state’s office.

“We will approve his application, which will probably happen tomorrow,” state GOP spokesman Spencer Yeldell said.

At the end of September, Stockman had just $32,000 in cash on hand for his House campaign committee — a paltry sum for a statewide race and one that he’ll have less than three months to improve upon before the Senate primary. Cornyn, meanwhile, had almost $7 million in the bank. Texas is one of the most expensive states to run a statewide campaign.

Stockman, a hardline conservative who pushed for the impeachment of President Barack Obama, has the capacity to rough up the second-ranking Senate Republican. The question is whether any outside groups will come in to support him financially.

“We all thought 10 minutes ago that this was a joke,” a state GOP operative who is not involved in either campaign told CQ Roll Call. “Based on Stockman’s history, Stockman will play nasty.”

The Cornyn campaign responded to the Stockman announcement by highlighting the senator’s conservative record, which includes a 93 percent lifetime score from the American Conservative Union and A grade from the National Rifle Association.

“Endorsed by Texas Right to Life and ranked as the 2nd most conservative Senator in America, Senator Cornyn looks forward to discussing his conservative record with Texans,” Cornyn campaign manager Brendan Steinhauser said in a statement.

Before Stockman’s announcement, a Cornyn aide said that during the senator’s recent back-to-back stints as chairman of the National Republican Senatorial Committee, Cornyn “always advised his members to be prepared, and he kind of took his own advice.”

Five other Republicans also filed to run, though none were likely to give Cornyn a legitimate scare. The primary is March 4.