His re-election campaign built up a war chest of nearly $7 million. He spearheaded a "Keep Texas Red" campaign to promote Republicans throughout the state. It looked at first if the strategy had worked, that potential challengers were intimidated enough not to oppose him.
But late Monday, on the last possible day for a Texas candidate to register for the Republican Senate primary, Rep. Steve Stockman, R-Texas, entered the race.
“Endorsed by Texas Right to Life and ranked as the second most conservative senator in America, Sen. Cornyn looks forward to discussing his conservative record with Texans,” said Brendan Steinhauser, Cornyn's campaign manager.
Stockman's office did not immediately respond to a request for comment.
In the era of Sen. Ted Cruz, R-Texas, who rocketed to an unlikely victory with the backing of conservative outside groups such as the Club for Growth, Cornyn's cautious preparation for a primary was to be expected. But an on-a-whim challenge from Stockman, who has faced fundraising difficulties even at the House level, was not.
"Stockman has taken absolutely no steps to prepare for his own re-election, let alone a statewide campaign of this magnitude," said one Republican strategist with ties to Cornyn. "He doesn't even have enough money to run radio ads in his own district, let alone a state with 23 media markets in a ... primary that's less than three months away."
Indeed, Stockman has only about $32,000 in the bank, with roughly $163,000 in outstanding debt and the primary election is March 4.
And Stockman can rule out getting any help from big-spending outside conservative groups, as Cruz did. The Club for Growth announced Tuesday that it does "not expect to be involved in the Texas Senate race."
“While Rep. Stockman has a pro-economic growth record, so does Sen. Cornyn, as witnessed by his 87 percent lifetime Club for Growth score,” said Club for Growth President Chris Chocola.
Another major conservative outside group, the Senate Conservatives Fund, has so far made no commitment to Stockman.
"We haven't decided yet whether we will endorse anyone in this race, but we're going to watch it closely," said SCF Executive Director Matt Hoskins.
Without substantial funding or a robust campaign organization, Stockman will likely face a range of challenges. His campaign headquarters in Webster, Texas, was abruptly shuttered last month for "multiple safety violations," according to the Houston Chronicle. And Stockman has made a host of controversial comments as a member of Congress, including when he said earlier this year, "If babies had guns, they wouldn't be aborted."
Still, Cornyn is not wildly popular: A recent survey by Public Policy Polling, a Democratic firm, found his approval rating to be roughly 46 percent, on the low side for an incumbent.
But Cornyn said Tuesday he welcomed a primary challenge.
"In the Democratic Party, the leadership picks the candidates and chases competitors out of primaries," Cornyn said. "We're much more of a bottom-up party."