The Tea Party group FreedomWorks is adding its name to the long list of conservatives backing Oklahoma House Speaker T.W. Shannon (R) against Rep. James LankfordJames Paul LankfordGOP senators raise concerns over tax plan Cornyn: Senate tax vote likely after Thanksgiving Senate GOP reveals different approach on tax reform MORE (R-Okla.) in their battle to succeed retiring Sen. Tom CoburnTom CoburnFormer GOP senator: Trump has a personality disorder Lobbying World -trillion debt puts US fiscal house on very shaky ground MORE (R-Okla.).

The group joins the Senate Conservatives Fund, Sens. Ted Cruz (R-Texas) and Mike Lee (R-Utah), and former Alaska Gov. Sarah Palin (R) in backing Shannon, who has quickly become anti-establishment Republicans' favorite in the race.

"T.W. Shannon is a principled leader who doesn’t simply talk about small-government reforms, he works to make them a reality," FreedomWorks President Matt Kibbe said. "We have enough talkers in Washington — it's time to send more leaders like T.W. Shannon who will get things done."

Lankford, a member of House GOP leadership who has compiled a conservative voting record in the House, has the backing of former Arkansas Gov. Mike Huckabee (R).

The primary is in late June.