House Budget Chairman Paul D. Ryan endorsed Idaho Rep. Mike Simpson on Tuesday, praising his fellow Republican’s work on debt reduction.  

Simpson, an eight-term incumbent and ally of Speaker John A. Boehner, is facing primary opposition from tea-party-backed attorney Bryan Smith in a marquee battle between competing wings of the GOP.  

“Mike Simpson has been in the trenches fighting to reform runaway spending and has been a critical voice in passing our plan to balance the budget and pay down our debt,” the Wisconsin Republican and former vice presidential nominee said in a statement.  

Ryan is the latest in a string of endorsements the Simpson campaign is rolling out, including from the Chamber of Commerce and 2012 GOP presidential nominee Mitt Romney . Smith’s primary bid is backed by the Club for Growth, which has made Simpson a top target.  

The winner of the May 20 primary will be heavily favored in November. The race is rated Safe Republican by Rothenberg Political Report/Roll Call.