Rep. Steve Stockman filed to run in next year's primary just under the deadline Monday and with no notice to Republican groups, Politico reported. Cornyn, the minority whip, who is seeking a third term, already had primary opposition from candidates unknown to most voters.
Stockman, 57, served a term in Congress between 1995 and 1997 and returned to the House this January. He has become known as an extreme conservative, comparing President Obama to Saddam Hussein and calling for his impeachment.
Cornyn holds what appear to be impeccably conservative views on most issues. But the Senate Conservatives Fund called him a "turncoat" earlier this year when he declined to support a procedural move by Sen. Ted Cruz, R-Texas, aimed at denying funding for Obama's Affordable Care Act.
A leader of the fund welcomed Stockman's entry into the race, although he declined to say whether the group would provide active support.
"We haven't decided yet whether we will endorse Steve Stockman, but we're glad he is running," Matt Hoskins said in a statement. "Texas deserves two conservative fighters in the Senate, not just one. John Cornyn has voted to increase the debt, raise taxes, bail out Wall Street banks, and fund Obamacare. He's part of the problem in Washington and voters deserve an alternative."
Stockman appears to have an uphill fight. His campaign owes thousands of dollars while Cornyn has a multimillion-dollar war chest.
Other senators facing challenges from the right include Minority Leader Mitch McConnell of Kentucky, Lindsey Graham of South Carolina, Thad Cochran of Mississippi and Pat Roberts of Kansas. Most are in heavily Republican states so the primary fights are unlikely to swing the seats to the Democrats, but the right-wing challenges are likely to make senators less willing to compromise with the other party.