A poll conducted for a group supporting former Oklahoma House speaker T.W. Shannon's bid for U.S. Senate shows him surging ahead of Rep. James Lankford (Okla.) in the Republican primary, whom he once trailed by a wide margin.

Shannon leads Lankford 42 percent to 32 percent among Republicans likeliest to vote, according to the Public Opinion Strategies survey. A third candidate, Randy Brogdon, lags behind. Pollster Glen Bolger notes in a memo that Shannon was down nine in March and 35 in February. The poll was conducted Monday and Tuesday for Oklahomans for a Conservative Future, a nonprofit supporting Shannon.

Shannon has received a wave of prominent tea party endorsements in recent weeks. He is backed by former Alaska governor Sarah Palin (R) and Sens. Ted Cruz (R-Tex.) and Mike Lee (R-Utah). The three are slated to appear at a Shannon campaign rally on Thursday.

A Lankford internal poll showed conducted in February showed the congressman up 30.

Shannon and Lankford are running for the seat being vacated by retiring Sen. Tom Coburn (R). The primary is June 24.