A serious primary challenger could defeat Trump — just not in the way you think

President Trump loves to boast of his popularity among Republican voters, and he is quite right that his approval ratings within his own party are consistently high. Support for Trump among self-identified Republicans has slipped below 80 percent for only a single week since he took office, and it regularly gets into the low 90s.

With numbers like these, the president has good reason to feel confident he'll win the GOP nomination again in 2020. However abysmal his broader approval rating and however small Republican voting ranks have become, these are the people who will vote in the GOP primaries, and they seem set to overwhelmingly pick Trump. Any primary challenger will almost certainly lose.

But that doesn't make such a challenge useless — far from it. Recent presidential history suggests a serious primary challenger, though doomed to fail, could guarantee Trump goes down, too.

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Since 1950, 12 incumbent presidents have sought re-nomination and re-election. (Trump in 2020 will be the 13th.) Of that dozen, only two were not nominated again: Harry Truman in 1952 and Lyndon Johnson in 1968. Both had low approval ratings within their own party — a mere 44 percent of Democrats approved of Truman heading into the New Hampshire primary, and Johnson barely scraped together majority support at 54 percent — which makes them poor comparisons for Trump, who is poised to go to New Hampshire with the second highest intra-party support in these seven decades.

More relevant are the three presidents in this period who were re-nominated but not re-elected: Gerald Ford, Jimmy Carter, and George H.W. Bush. These three also had lower primary season approval ratings within their parties than Trump has now, but the difference is much less dramatic, with all three pulling at least two-thirds support.

Ford, Carter, and Bush the elder were the only sitting presidents in those 70 years to lose a general election, and they were also the only three nominees to be seriously primaried. As Perry Bacon Jr. and Dhrumil Mehta note at FiveThirtyEight, there are two possible explanations: "It's not totally clear which way the causation runs here — did the primary challenge weaken Ford, Carter, and H.W. Bush ahead of the general election, or was it simply a symptom of a weakness that already existed?"

Bacon and Mehta argue for the latter, and they're probably correct. But there's no reason to think this is an either/or proposition. Trump is and likely will remain more popular within his own party than these three presidents were, but a credible primary challenger could still do real damage to his general election chances.

A president who can cruise through primary season unchallenged is a president who can conserve his war chest for the main event. He is spared the scrutiny of primary debates, freed to launch broadsides across the aisle while his potential opponents must battle among themselves. He can avoid reminding his own party's voters that there are actually things — maybe a lot of things — they don't like about him, policy positions or personal failings another candidate might not have. He can stick to a single strategy and message for all 7 billion months of our endless campaign season, jumping straight to a focus on the general election instead of doing the post-primary pivot.

A President Trump who doesn't have a serious primary challenger has it easy.

Yet a lot hinges on one word: serious. For a primary challenger to have a real effect on Trump's general election standing, he or she would have to mount an impressive campaign even while knowing it is, in the immediate sense, all for naught — and very possibly the end of this person's political career. This would require a strong fundraising operation and the established name recognition of a Mitt Romney, John Kasich, or maybe Jeff Flake.

Romney and Kasich in particular are among the few it is possible to imagine having the history and clout in the GOP to insist Trump be compelled to participate in primary debates. That seems much less likely with Bill Weld, the one potential primary challenger to officially announce his exploratory committee to date. Weld is a former Republican governor of Massachusetts, but he also campaigned for vice president with the Libertarian Party in 2016, bizarrely seeming more enthusiastic about the candidacy of Hillary Clinton than that of his own running mate.

So Weld is probably not the challenger to get Trump to the debate stage and busy him during primary season, but someone else could do it. They would be in for a terrible slog marked by juvenile name-calling and one defeat after another. They would not become president, and they might never hold any elected office again. They could even play a role in helping elect a Democratic candidate whose administration they would find nearly or equally as objectionable as Trump's.

But if the aim is to get Donald J. Trump out of the White House, a serious primary challenger may be just the ticket.