Rep. Mark Sanford (R-SC) is the lone outspoken critic of President Donald Trump among House Republicans — but he wishes his colleagues, especially Speaker Paul Ryan, would perform their constitutional duty to check the chief executive’s power.

The South Carolina Republican, whose own presidential ambitions were derailed by an infamous extramarital affair in 2009, admits he’s not the most credible critic of another man’s honesty, but he told Politico that Trump’s pattern of dishonesty violated the nation’s founding principles.

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“You want to give anybody the benefit of the doubt,” Sanford said. “I mean, I’ve learned that through my own trials and tribulations.”

MARK SANFORD, whose district Trump is visiting today, lays waste to the president in an extraordinary interview: https://t.co/9A0pEFB0ea — Tim Alberta (@TimAlberta) February 17, 2017

“But if you see a pattern of over and over and over again, wherein facts don’t matter and you can just make up anything,” Sanford said, pausing to gather his thoughts. “Our republic was based on reason. The Founding Fathers were wed to this notion of reason. It was a reason-based system, and if you go to a point wherein it doesn’t matter, I mean, that has huge implications in terms of where we go next as a society.”

Sanford, who received a barrage of criticism from conservatives after calling on Trump to release his tax returns during the campaign, told Politico he’s troubled that the president “has fanned the flames of intolerance” and repeatedly misled the public.

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“All of a sudden a guy comes along where facts don’t matter?” Sanford said. “It’s somewhat befuddling. It’s the undoing of that which you base a large part of your life on.”

The conservative Christian also expressed dismay over Trump’s lack of humility at the National Prayer Breakfast, where the president mocked Arnold Schwarzenegger, his successor as star of “The Celebrity Apprentice.”

“I believe in a war of ideas … and I tell the staff all the time: Look, we’re in the business of crafting and refining our arguments that are hopefully based on the truth,” Sanford told the website. “Truth matters. Not hyperbole, not wild suggestion, but actual truth.”

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Sanford understands the historical deference toward the presidency from within his own party, and he appreciates that his fellow Republicans are afraid of an executive who “has a proven record of taking people down.”

But he’s disappointed that Ryan, the house speaker, seems unwilling to challenge Trump on even their most basic political differences.

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“I admired his conviction in the campaign,” Sanford told Politico. “(But) at the end of the day, radio silence is not sustainable in being true to yourself.”