Mark Sanford, former Republican governor and representative of South Carolina, is considering a primary challenge to President Trump.

True to style, Sanford — who was known as a stringent fiscal conservative ("famously cheap") before he was known as the guy with the Appalachian Trail tale — linked his campaigning impulse to economics. "I'm a Republican. I think the Republican Party has lost its way on debt, spending, and financial matters," he told Charleston's Post and Courier. "Sometimes in life you've got to say what you've got to say, whether there's an audience or not for that message."

As Sanford seems to realize, in the Republican primaries for 2020, there probably is not much audience for his message. Though far less popular with the electorate at large, Trump's approval rating is consistently high with his loyal partisan base. GOP support for Trump has hovered around 90 percent all this year; his primary victory is all but guaranteed. Sanford almost certainly won't win — but he should run anyway.

This is not about viability. After 2016 we should all be wary of absolutist electoral predictions, but it would be quite the surprise if Sanford could muster a serious threat to Trump's re-nomination. What he can do, however, is run a serious campaign.

In an open primary season, Sanford would be a credible contender. His political résumé is respectable, and until Trump opposed him in the 2018 primaries, he'd never lost an election. His comeback to the House helped put his history of scandal behind him, and, back in Washington, he built a reputation as one of exceedingly few Republicans willing to criticize the president. A 2020 Sanford campaign is doomed, but not because of candidate Sanford.

So why run anyway? Because the GOP field needs warm bodies. A single challenger — currently, former Massachusetts Gov. Bill Weld (R) — is unlikely to be able to force the Republican Party to run a primary contest. Though he too could be taken reasonably seriously in an open primary because of his past office, Weld alone cannot get Trump to a debate stage. Sanford and Weld together probably can't either. But with a large enough slate of plausible candidates, eventually GOP leadership would have to respond.

There is a tipping point here. I don't know what the tipping point may be, but I think it does exist. (If I had to hazard a guess, I'd say it's somewhere above two good candidates but below 10, and that the addition of names with national recognition, like a Mitt Romney or John Kasich, could well lower the goal.) The more credible candidates join the race, the better chance we have of reaching that line. Sanford doesn't need to have a substantial Republican audience for his message to make the campaign trail a good venue for saying what he's got to say.

And Sanford is arguably in an ideal position for growing the 2020 field. He's "been dead politically," he told Politico in 2017, and "[i]f you've already been dead, you don't fear it as much." That fearlessness allowed Sanford to freely censure Trump in a way nearly no other Republicans have been willing to do. He mocked Trump for his ignorance of the Constitution; slammed him for "fann[ing] the flames of intolerance;" and castigated him for helping to normalize violence against political foes. This week he called Trump's racist tweets about minority members of Congress "noxious," "inflammatory," and "weird." Rebukes like these make for dramatic debates — ironically a potential draw to our reality TV president, perpetually spoiling for a fight.

And that fight is exactly why a GOP field large enough to drag Trump to the debate stage is worth the critical mass of challengers it would require: The president's primary win may be inevitable, but he should be made to work for it. His vision for the country should be interrogated from the right as well as the left, as it will be in the general.

A 2020 race between Trump and Sens. Elizabeth Warren (D-Mass.) or Bernie Sanders (I-Vt.), for example, would have little substantive debate on trade policy even as this administration's protectionism wreaks havoc in industries like agriculture, dairy, and steel. Republican primary debates would showcase critiques of more than Trump's execution of his tariffs plan.

More broadly, the president may have permanently remade his party already. The GOP may indeed be entirely done with the free market economics we'd hear from a fiscal conservative like Sanford or the social moderation of a Rockefeller Republican like Weld. Yet if there is any chance the Trumpian transformation isn't an indelible shift, a diversity of Republican voices in the 2020 race might remind voters of how different the GOP still could be.