The conservative group Citizens United is endorsing Minnesota state Sen. Julianne Ortman's (R) Senate campaign, bucking establishment Republicans who want businessman Mike McFadden (R) to lock up the nomination against Sen. Al FrankenAl FrankenCritics eye repeal of ObamaCare prescription drug tax DeVos doesn’t know the ABCs of public education Franken emerges as liberal force in hearings MORE (D-Minn.).

"Julianne Ortman believes in individual liberty, limited government, and the sacred rights set forth in the U.S. Constitution," said Citizens United President David N. Bossie.

Bossie went on to attack the GOP establishment for backing McFadden.

"Minnesotans deserve a clear choice — and Julianne Ortman can best articulate that choice because liberal Mike McFadden is just 'Franken-lite.' Unfortunately, the National Republican Senatorial Committee and other Washington establishment fixtures are supporting McFadden as if Minnesotans have already made their decision," he said.

"If Julianne Ortman wins the Republican Party's endorsement at the upcoming convention in May, it’s imperative that Mike McFadden honor the tradition and rally behind Ortman's candidacy."

McFadden has said he'll stay in through the primary no matter the outcome of the party convention.

McFadden has a huge fundraising advantage over Ortman in the race, but a poll conducted last month for Citizens United found a wide-open primary, with her in the lead.

Franken is favored against his opponents in the fall.