Former state senator Randy Brogdon, a tea party-aligned candidate who took 39 percent against now-Gov. Mary Fallin (R) in the 2010 primary, was not included in the poll because he wasn't a Senate candidate yet.
As always, polls conducted for campaigns should be viewed with a degree of skepticism. But Lankford has been viewed as the favorite in the race since he got in -- in large part because no other members of the state's congressional delegation opted to run and because tea party groups have yet to rally around an alternative.
The poll shows Lankford starts out with solid numbers, with 45 percent of Republicans having a favorable impression of him and just 5 percent having an unfavorable one.
Shannon, Lankford's main competition, is less well-known, with a 26 percent favorable rating and 5 percent unfavorable.
Despite his conservative record, groups like the Club for Growth do not like Lankford -- who is the fifth-ranking member of House GOP leadership as Republican Policy Committee chairman -- and have made little secret of that fact. The Club is set to make some kind of Senate race endorsement on Wednesday, but it's not clear whether it will be in Oklahoma.
Shannon has attracted some early attention as a rising star in the GOP. The 36-year old is the first African American speaker of the Oklahoma state House and also has
Cherokee Chickasaw heritage in a state with a sizable American Indian population.
If tea party groups mount a challenge to Lankford, they'll have some work to do. Sixty percent of those who know Lankford say he is a conservative; just 39 percent say the same about Shannon.
The poll was conducted Feb. 10-12 among 500 likely Republican primary voters. The margin of error is plus or minus 4.9 percent.
Sen. Tom Coburn (R-Okla.) is retiring. Democrats are not expected to compete in the general election and have yet to recruit a candidate. Top Democrats have opted not to run.
The primary will be held June 24, with a runoff possible if no candidate gets 50 percent of the vote.