When the Republican candidates for president debate Tuesday night, they should all be asked a simple question: In light of Donald Trump’s crude and divisive stances, will you refuse to support him if he is the party’s nominee for president?

Only by making it clear that some principles matter more than partisanship will the Republican contenders give meaning to their recent criticisms of the front runner.

Unfortunately, Republican candidates and party leaders have so far been unwilling to reject Trump and Trumpism in anything more than the most self-serving and frequently tepid terms. Plenty of Republicans say they object to Trump’s religious-test bigotry, and a few have even objected to his other bigotries. But they lack the courage to declare that they will not support the billionaire if he secures the Republican nomination.

The worst of the lot, House Speaker Paul Ryan, says Trump’s proposal to bar Muslims from the United States is “not conservatism” and “not what this party stands for,” and then says that, of course, “I’m going to support whoever the Republican nominee is…” Translation, he will call out Trump, collect compliments for raising concerns, and then back Trump.

Ryan, party leaders like Republican National Committee chair Reince Priebus, and most of billionaire’s fellow contenders for the 2016 GOP nomination strengthen the hand of the Trump and Trumpism when they distance themselves from the most noxious expressions of his politics, yet say they plan to back him if he is nominated.

The only way to block Trump is to oppose Trump, clearly and unequivocally.

And, as of now, very few Republicans are doing that.

One congressman is stepping up, however, to unequivocally reject Trump. And in so doing he is showing his fellow Republicans how to stand on principle.

Wisconsin Republican Reid Ribble, who represents a historically competitive district just north of Ryan’s, is saying absolutely and unapologetically that he does not back Trump—and, more importantly, that he will not back Trump.

“I am not obligated to support a bad candidate from any party,” says Ribble. “I will not support Donald Trump for president of the United States, no matter what the circumstances.”

Ribble has served three terms in the House and has developed a reputation as a common-sense conservative with an independent streak. Earlier this year, he quit the right-wing Freedom Caucus, after it disrupted the process of replacing former Speaker John Boehner.

To a greater extent than any Republican member of Congress—including South Carolina Senator Lindsey Graham, the most outspoken of Trump’s foes for the party’s presidential nod—Ribble has objected to the billionaire’s sexism and bigotry.

Last summer, Ribble told a Wisconsin radio station, “I reject wholeheartedly the Trump campaign for president. I think it works at our most base interests, it’s prurient.”

In September, he ripped on Trump in a conversation with USA Today, saying, “It’s not politics what he’s doing, it’s a carnival. It would be one thing if he was a serious policy person, but he’s not.”

“You can’t be calling women bimbos, we can’t just be kicking sand in the sandbox saying, ‘You’re dumb’ and ‘You’re a loser,’” the congressman added. “We actually need a grown-up, not a three-year-old in the White House.”

Ribble called Trump’s immigration plan “10 or 15 pages of blather.”

He got that right.

And he gets it right when he recognizes that the only way to oppose Trump is by refusing to back Trump.

Ribble pulls no punches, telling the Milwaukee Journal Sentinel that he objects to Trump’s determination “to be as inflammatory as possible, which appeals to the worst parts of who we are as people. It appeals to our fears. It appeals to our racism. It appeals to all the negative things about us.”

Though he says he understands the “natural caution” of party leaders and presidential contenders when it comes to calling out Trump, Ribble is actually doing what party leaders must do. He is drawing a clear line of distinction between his vision of Republicanism and Trumps, saying, “I don’t want to offend anyone that’s supporting Mr. Trump. They’ve got their own reasons for supporting him. I’ve got my reasons for not.”

Ribble is blunt about those reasons, explaining that Trump “has offended women, he has offended African-Americans, he has offended Muslims. It’s just one thing right after another…the way he talks, the language that he uses.” He is also blunt about his belief that Trump is a political charlatan. “I do believe that many of Mr. Trump’s supporters are looking for the ‘anti’—the anti-Obama, the anti-Washington,” the congressman told the Journal Sentinel. “They believe he’s speaking the truth [but] no, he’s really not.”

This is the discussion that has to be had with the party base. Ribble is ready to engage in it. And he is ready to see it through to the only logical conclusion: declaring that he will not back Trump. Only by doing this will Republicans who object to Trump be taken seriously. And only if Republicans who object to Trump are taken seriously will those objections be seen as anything more than the empty rhetoric of political careerists whose principles will always be sacrificed to self-interest and partisanship.