Longtime Texas Sen. John Cornyn, R, appears vulnerable to a primary challenge from conservative firebrand Ken Paxton, the current state attorney general, a recent poll has shown.

Paxton led Cornyn in a one-on-one match up, with 50% of Republican primary voters backing him, to the incumbent's 28%, in a recent UpOne Insights survey.

The attorney general increased his lead in what the poll called an "informed ballot test." In that scenario, voters were told to consider a prospective Trump endorsement of Cornyn, Paxton's likely criticisms of Cornyn, and Cornyn's record on hot-button conservative issues like gun rights.

In that scenario, Paxton led 62% to 21%.

Cornyn has long ranked among the Senate GOP's more moderate members, while Paxton is widely regarded as a conservative heavyweight. He successfully survived an impeachment effort spearheaded by then-state Speaker of the House Dade Phelan, who subsequently lost his leadership post.

Conducted May 27-28, the survey questioned 600 Republican primary voters and has a margin of error of +/-4%.