Nebraska Sen. Ben Sasse drew scorn and even threats from Donald Trump's supporters after he tweeted a series of critiques and questions about the Republican presidential front-runner in January.

Undaunted, the freshman GOP senator posted an online video in which he read aloud a litany of "mean tweets" about himself. Wearing a red University of Nebraska pullover, the boyish-looking Sasse recited rebukes calling him a loser, "a joke looking for a punchline" and asking how "this bozo is a U.S. senator."

Sasse laughed off the insult. "My brother says this every week," he joked.

Now Sasse again finds himself tangling with Trump and his supporters after he became the first Republican senator to declare publicly he would not back Trump if the billionaire businessman wins the GOP nomination for president.

A Nebraska state senator called Sasse's comments "very immature," while Trump dismissed the suggestion that conservatives should consider backing a third-party candidate.

"That would be the work of a loser," Trump told reporters on Super Tuesday.

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Sasse, 44, said in an interview that he takes the criticisms seriously, but believes he has an obligation to speak out.

"The vast majority of people supporting Donald Trump are well-meaning people who are angry about the direction of the country and feel we in Washington aren't urgent about solving the problems," said Sasse, a former college president who was elected in 2014 with support from the tea party.

"People want to say no to Washington. I agree with that and support it," the first-term senator said. "But there's a difference between being against what's wrong with the country and truly being for Donald Trump."

Many Nebraska Republicans took issue with Sasse's remarks.

State Sen. Bob Krist of Omaha, who called the comments immature, said Sasse needs to realize he's no longer a professor or college president.

"This is one of those times that you quietly and in a statesman-like manner allow the system to work out and provide the leadership that needs to be provided," Krist said.

State Sen. Beau McCoy, also of Omaha, said he struggled to see how Sasse's statements help Republicans.

"It is a little interesting and peculiar when we're looking at winning in November," said McCoy, who said he will support whoever the GOP nominates for president.

   Sasse, who does not face re-election until 2020, said he knows his remarks are unpopular in his strongly Republican state. But he called Trump "a guy who wants to tear things down and divide Americans" and said he sees a resemblance between Trump and Democrat Hillary Clinton -- "two fundamentally dishonest New York liberals."

   Sasse is an unlikely outsider. He has degrees from Harvard and Yale and worked in the Justice Department and as an assistant secretary of Health and Human Services under President George W. Bush before serving as president of Midland University in Nebraska.

   Yet he is known for ruffling feathers and scrapped with Kentucky Sen. Mitch McConnell during his 2014 campaign. McConnell, now the Senate majority leader, took umbrage at Sasse's comments questioning taxpayer subsidies for health care for members of Congress.

Sasse raised eyebrows again in 2015 when he asked, in his first speech on the Senate floor, why the Senate exists and what would be lost if the nation didn't have it. Sasse answered his own question by saying the Senate's purpose is to shield lawmakers from "an obsession with short-term popularity so we can focus on the biggest long-term challenges we face."

His criticism of Trump is consistent with that view, Sasse said. While many Nebraskans have denounced him, Sasse said he is convinced he is serving the state and the country by calling out Trump as a "creepy" egomaniac who "thinks he's running for king."

If Trump is the party's nominee, "my expectation is that I'll look for some 3rd candidate -- a conservative option, a constitutionalist," Sasse wrote on Twitter.

Fellow Nebraska Republican Sen. Deb Fischer strongly disagrees.

"I believe a third-party candidate would give us another President Clinton. We've seen that story before," Fischer said, contending that Ross Perot's independent bid helped Bill Clinton win the presidency in 1992.

Fischer, who backs Florida Sen. Marco Rubio for president, said Sasse's comments are unlikely to have much impact in the Senate. "He's one senator out of 100 and that's his opinion," she said.

Still, Fischer praises Sasse as a hard worker and bedrock conservative who focuses on the needs of his constituents. "We send each other text messages and root each other on," she said.

Sen. John McCain, R-Ariz., sees Sasse as a rising star.

"I think this is a man of conscience," McCain said.

Sen. Richard Burr, R-N.C., called Sasse "a brilliant addition to the Senate," but said his comments were unlikely to persuade other senators to disavow Trump.

Even Sen. Lindsey Graham, a South Carolina Republican who has called Trump a "nut job," questioned who would be a viable third-party candidate.

"I'd like to keep the Republican Party in a position to win in 2016," Graham said.