Since Donald Trump became the presumptive Republican nominee Tuesday night, much of the official GOP has moved—with varying degrees of enthusiasm—to get behind him.

But a few #NeverTrump holdouts remain. And Ben Sasse, a freshman senator from Nebraska, has emerged as their unlikely leader. For months, Sasse has been calling for a third-party conservative presidential candidate should Trump be the Republican standard-bearer. And in a lengthy “Open Letter” posted to Facebook Wednesday, as well as a multi-part Twitter storm, he doubled down on that stance.

“With Clinton and Trump, the fix is in,” Sasse wrote. “Heads, they win; tails, you lose. Why are we confined to these two terrible options? This is America. If both choices stink, we reject them and go bigger.”

Sasse, 44, has made clear that he won’t be that bigger option, citing his young kids. But he figures to play a leading role in any effort to run a conservative alternative to Trump—a project that could further undermine the bombastic billionaire’s chances of winning the White House. And Sasse is already being talked about as a potential leading GOP presidential candidate in 2020.

So, who is Ben Sasse?

A fifth-generation Nebraskan, Sasse was his high-school valedictorian and has degrees from Harvard and Yale. For good measure, he did his junior year at Oxford University.

Before running for the Senate in 2014, Sasse had a varied resume. He worked as a management consultant for top firms like Boston Consulting Group and McKinsey; as chief of staff to a Republican congressman; as a top staffer at the Bush Justice Department’s Office of Legal Policy; as a high-ranking official at the Department of Health and Human Services; and as the president of a private Nebraska college, Midland University.

OK, but what does he believe?

Sasse is adept at the kind of “pox on both their houses” rhetoric that positions himself as above politics and drives some Beltway pundits wild.

“We’re not doing the job we were sent here to do,” he lamented in his maiden Senate speech last year. “The Senate isn’t tackling the great national problems that worry those we work for.”

But he’s a rigid, Ted-Cruz-style ideological conservative who supports privatizing Social Security, opposes abortion rights and same-sex marriage, and wants to “rip apart” Obamacare.

Perhaps not surprisingly, he’s been backed by Tea Party favorites like Cruz, Sarah Palin, and Sen. Mike Lee of Utah, who held a rally for him during that campaign. Paul Ryan is also a longtime fan. And he’s beloved by Washington’s conservative intellectual class: National Review and The Weekly Standard both published glowing profiles of him before he was even the GOP’s Senate candidate in 2014 (the latter called him “a virtuoso pol”).

Like Cruz, Sasse has even taken on Mitch McConnell, the GOP Senate Leader. “It is time for every Republican in Washington, starting with Minority Leader Mitch McConnell, to show some actual leadership on this issue,” Sasse said during the 2014 campaign, calling on Republicans to give up their health-care subsidies. But the two appear to get along fine these days. Friends of Sasse have reportedly urged him to go easy on McConnell since he got to the Senate, and the GOP leader sent the text of Sasse’s maiden speech to every member of his caucus.

The genial Sasse wins plaudits for his smarts and honesty even from those who’d never agree with him. “He’s absolutely a free-market conservative,” Howard Dean, who has held public debates with Sasse over health care, has said. “I wouldn’t vote for him, but he sticks to the truth and makes good, tight arguments for conservative health-care reform.”