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Republican Sen. Ben Sasse used social media Sunday to say he would not vote for Donald Trump

"I'm as frustrated and saddened as you are about what's happening to our country. But I cannot support Donald Trump," Sasse wrote

Washington CNN  — 

Republican Sen. Ben Sasse said Sunday he won’t vote for Donald Trump if the billionaire and GOP presidential front-runner becomes his party’s nominee.

“If Trump becomes the Republican nominee my expectation is that I’ll look for some 3rd candidate – a conservative option, a Constitutionalist,” Sasse, a first-term senator from Nebraska, tweeted Sunday night.

Sasse railed against Trump on social media, writing in one tweet that “The Presidency is not our national embodiment of Nietzschean Will.”

He collected many of his concerns in a Facebook post.

“I’m as frustrated and saddened as you are about what’s happening to our country. But I cannot support Donald Trump,” Sasse wrote. “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.”

Sasse, who has not made a formal endorsement for a 2016 candidate this cycle, has campaigned against Trump in the past, and appeared with Sens. Ted Cruz and Marco Rubio on the trail.

The Nebraska freshman reiterated his position an interview Monday with CNN’s Jake Tapper on “The Lead.”

“If the nation that put a man on the moon can’t do any better than nominating two fundamentally dishonest New York liberals, I think the American people deserve better choice than that and I think they’ll get a better choice,” Sasse said of Trump and Clinton.

“I’m not going to vote for Hillary Clinton, and given what we know about Donald Trump, I can’t vote for that guy either … If we got to a place where those are the two major party nominees, and I certainly hope that they’re not, I’d have to look for a third-party option,” he told Tapper.

CNN’s Eric Bradner contributed to this report.