Rep. Paul Ryan (R-Wisc.) has endorsed Nebraska Republican Senate candidate Ben SasseBenjamin (Ben) Eric SasseSenators push foreign media to disclose if they are registered as foreign agents Beijing: US 'oppressing Chinese companies' after Huawei, ZTE action Senate Republicans defend Trump's response on Russian bounties MORE in what's becoming an increasingly contentious primary for retiring Sen. Mike Johanns' (R) seat.

"Ben Sasse is a true conservative who will help America move forward," Ryan said in a statement issued Monday. "He strongly believes in our founding principles. And he fully understands the need to repeal ObamaCare and offer real, patient-centered reforms. That's why we need him in the Senate."

The Midland University president has gained the backing of a number of other national conservative groups, including the Senate Conservatives Fund and the Club for Growth.

But one of his primary challengers, former state Treasurer Shane Osborn, has the endorsement of another prominent group, FreedomWorks.

Sasse was reportedly criticized by Senate Minority Leader Mitch McConnell (R-Ky.) in a private meeting for his association with SCF and for a video he posted in which he called for every "Republican in Washington, starting with Minority Leader Mitch McConnell, to show some actual leadership."