The Senate Conservatives Fund on Thursday endorsed former Oklahoma state House speaker T.W. Shannon (R) for the seat of retiring Sen. Tom Coburn (R-Okla.), wading into yet another a contested Republican primary.

"T.W. Shannon is a constitutional conservative who will fight to stop the massive spending and debt that are bankrupting our country," said Senate Conservatives Fund executive director Matt Hoskins. "T.W. Shannon believes in the principles of freedom that make this country great and will stand up to the big spenders in both parties to balance the budget and stop Obamacare."

Shannon, the first African American to serve as House speaker in Oklahoma, recently won the endorsement of former Alaska governor Sarah Palin (R). He is running against Rep. James Lankford (R). Early polling has shown Lankford is the frontrunner.

Senate Conservatives Fund has made endorsements in a handful of other Senate races. The group is backing Republican challengers against GOP senators in Kansas, Kentucky and Mississippi.