It was the last straw for the junior senator from Nebraska. When Republican presidential candidate Donald Trump “played footsie” with Ku Klux Klan grand wizard David Duke on a Sunday talk show last month, the GOP lawmaker says he had to act.

That night, Sen. Ben Sasse posted an open letter on his Facebook page to supporters of Mr. Trump, saying that he would not support him if he were the nominee, nor would he swing over to Hillary Clinton. He would instead seek out a third candidate – a conservative, a constitutionalist.

Senator Sasse (pronounced “sass”) blasted Trump for dividing Americans and acting like a monarch. “Have you noticed how Mr. Trump uses the word “Reign” – like he thinks he’s running for King? It’s creepy, actually.”

The letter was politically risky, given Trump’s popularity and Sasse’s status as a freshman senator – 99th in seniority. Nebraska’s senior senator, Republican Deb Fischer, disagrees with his third-candidate strategy. Meanwhile, the GOP leadership in both the House and Senate say they will support Trump if he's the nominee.

But the move was typical Sasse, an independent-minded senator who is willing to go against the grain in Washington.

The Nebraskan believes the nation’s leaders ought to focus urgently on a few big things – national security, the debt, the economy – and warns of a “constitutional crisis” of presidential overreach. He ranks as the fourth most conservative member of the Senate, according to one voting scorecard.

But he pans the partisan screaming, the straw-man arguments, the “fake budgets” and non-urgent side issues. It’s time for a more constructive politics, he argues.

This fascinates Sen. Susan Collins (R), a moderate from Maine who was recently ranked the most bipartisan in the Senate, according to an index by The Lugar Center.

“I’m very intrigued by him, to tell you the truth, because he’s not easily categorized, and he’s not predictable,” she said in an interview on the Senate subway as it zipped her to her office. “He’s very conservative, but he’s not reflexively for one position or another. He’s gutsy, as [his] speeches certainly prove. There’s just a real solid core to him that I find very appealing.”

Far from isolating him, Sasse’s forthrightness, fired by an intellect that’s certified in five diplomas, has won him respect and admiration from colleagues on both sides of the aisle. He’s a tea-party favorite without the explosives. A conservative who decries bare-knuckled politics as a substitute for principled governing.

“He’s a rising star,” says fellow Republican Sen. John McCain (R) of Arizona.

Democrat Heidi Heitkamp of North Dakota, who serves on several committees with Sasse, describes him as “thoughtful” and “very willing to listen to an argument."

Sasse did not follow the usual route to the Senate that travels through state and local offices or through the US House. A fifth-generation Nebraskan, he lives in his home town of Fremont, with his wife and three children, who have been home schooled. His dad was a high school teacher and coached football and wrestling there, and the young Sasse was recruited to wrestle at Harvard. He later earned a PhD in history from Yale.

Most of his career has been as a strategic consultant, starting at Boston Consulting Group. He helped companies steer through crises in the utility, telecom, and airline industries.

His last job was as the president of Midland University in Fremont, where he was brought in to reverse course at the financially troubled liberal arts school that is affiliated with the Evangelical Lutheran Church. One of the youngest college presidents in the nation, he abolished tenure, cut back underperforming departments while building up others, and concentrated on recruiting.

But how does a guy who’s a turnaround artist make an impact as 1 of 100?

The Senate is a long game, and Sasse has just one year under his belt. Speeches are one thing, but it can take years to work an important piece of legislation through the gates of a committee, onto the Senate floor, past a possible filibuster, through the House, and onto a president’s desk for signature.

In a phone interview, Sasse says his experience as a university president is the most analogous to his new job, because it’s not command-and-control. Faculty members have their own domain, and as president, you’re more like a symphony conductor bringing out the richness of the group. He laments the lack of leadership in Washington and of a shared understanding and celebration of the “American idea.”

At the same time, the Senate is brand new to him, and for that, he’s had to put his historian’s hat on. As he worked his way up to his maiden speech last year, he kept a low profile, interviewing senators about what worked and didn’t. He studied up on the ways and rules of the Senate, and has been in touch with the Senate historian’s office.

On his office desk right now are eight books, including a volume of addresses on the history of the Senate by the late Sen. Robert Byrd (D) of West Virginia – a master of Senate rules and history – and “Shattered Consensus,” by James Piereson, about the rise and decline of America’s post-war political order.

Former Senate historian Don Ritchie describes Sasse as “off to a good start,” quietly observing the institution. “They are all in a hurry these days, and the Senate requires a certain amount of time to be heard, and to be listened to.”

Traditionally, senators develop areas of expertise as a way to make their mark, says Mr. Ritchie.

The boyish-looking senator appears to be doing just that. He is spending his energy on some big, long-term issues such as cyber-security in an age of terrorist jihad and also health care.

This week his office released a report saying that the Obamacare exchanges are not as competitive as the administration claims. He has also held fire to the administration’s feet over the Affordable Care Act’s troubled “co-op” program.

Sasse’s focus on health care policy was evident even before he got to the Senate.

Alongside his private sector and university activities, he had done stints in Washington before (the Justice and Health departments under President George W. Bush, and working for fellow Nebraskan Rep. Jeff Fortenberry before that). But his time in Washington didn’t hurt his anti-establishment image among Nebraskans.

In 2014, he ran a fiercely competitive campaign, backed by three major conservative groups – Club for Growth, FreedomWorks, and the Senate Conservatives Fund. Senate Republican leader Mitch McConnell (R) of Kentucky, who was trying to swamp tea party upstarts, backed someone else. Sasse won every single county, putting replacement of the Affordable Care Act at the center of his campaign (his plan totaled more than 20 pages).

He hasn’t succeeded in that quest yet. But he has already turned heads with his plain-spokenness and intellect.

Last November, a year after being elected to his first political office, the student of history held up the mirror to his Senate colleagues in a startling maiden speech.

“The people despise us all,” he said in remarks that invoked Senate greats and Socrates. He urged the Senate to return to substantive debate and tackle America’s big problems.

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Some colleagues see Sasse as part of a larger shift under way in the Senate.

“The talent, the commitment, the knowledge, the broadness of our members is much better today than it was nine years ago,” says Sen. Bob Corker (R) of Tennessee, speaking of when he first entered the Senate. “I look at Sasse as one of these people,” says the Foreign Relations chairman, who recently reread Sasse’s maiden speech to help him with one of his own. “He’s just extraordinarily bright and insightful.”