Paul Singer

USA TODAY

Nebraska Republican Sen. Ben Sasse said Sunday night that he will not vote for Donald Trump if the billionaire wins the Republican nomination and will instead seek "some third candidate — a conservative, a Constitutionalist" to support instead.

In a lengthy Facebook note posted late Sunday, Sasse argued that Trump does not appear to support the constitutional principles most dear to conservatives, such as limited government and First Amendment protections for religious liberty.

"Much like President Obama, he displays essentially no understanding of the fact that, in the American system, we have a constitutional system of checks and balances, with three separate but co-equal branches of government. And the task of public officials is to be public 'servants,'" Sasse wrote.

"I’m as frustrated and saddened as you are about what’s happening to our country. But I cannot support Donald Trump. Please understand: I’m not an establishment Republican, and I will never support Hillary Clinton. I’m a movement conservative who was elected over the objections of the GOP establishment. My current answer for who I would support in a hypothetical matchup between Mr. Trump and Mrs. Clinton is: Neither of them. I sincerely hope we select one of the other GOP candidates, but if Donald Trump ends up as the GOP nominee, conservatives will need to find a third option."

Sasse, with the endorsements of Ted Cruz and Sarah Palin, beat two better-known Republicans in the GOP primary in Nebraska in 2014, in what was considered a win for the Tea Party wing of the party.

The freshman senator's announcement comes as other high-profile Republicans are beginning to line up behind Trump, the clear front-runner for the GOP nomination.

Sen. Jeff Sessions, R-Ala., one of the chamber's most outspoken opponents of illegal immigration, endorsed Trump at a rally Sunday, becoming the first U.S. senator to back him. New Jersey Gov. Chris Christie endorsed Trump on Friday.

Sasse has been campaigning against Trump for weeks and has repeatedly taken to Twitter to attack Trump for everything from his positions on the Constitution to his history of broken marriages to his performance in the debates.