Mark Sanford, the former representative and governor of South Carolina, has now joined former representative Joe Walsh and former Massachusetts governor Bill Weld in challenging President Trump for the 2020 Republican presidential nomination.

Of course they have no chance. But the hope of some Democrats and NeverTrumpers is that a primary challenge will weaken the president enough that he will lose to his Democratic opponent in the general election.

Trump adversaries often note that no president who has faced a significant primary challenge in the last 50 years has gone on to win re-election. They point to President George H.W. Bush, who lost in 1992 after a primary challenge by Pat Buchanan. To Jimmy Carter, who lost in 1980 after a primary challenge by Ted Kennedy. To Gerald Ford, who lost in 1976 after a primary challenge by Ronald Reagan. And to Lyndon Johnson, who withdrew in 1968 after a primary challenge by Eugene McCarthy and Robert Kennedy.

How can Donald Trump have a chance to win in 2020, now that he is facing challengers of his own? The answer is that there are primary challenges and then there are primary challenges. To say the least, there is a significant stature gap between Sanford-Walsh-Weld and the challengers of the past. Robert Kennedy, Ronald Reagan and Ted Kennedy were major political figures at the height of their careers when they decided to take on sitting presidents. Buchanan was a well-known White House aide, commentator, television personality and all-around legend among conservatives.

Sanford, Walsh and Weld are all former officeholders whose best years in politics are behind them.

"Let me ask you something," Buchanan told me in a recent conversation. "If Trump were not running in 2020, how would Joe Walsh and Bill Weld and Mark Sanford do in the New Hampshire primary? They would do nothing. Their calling card is, we can't stand Trump and he ought to be thrown out. If that's all it is, it's wholly negative." Buchanan stunned Bush in New Hampshire in February 1992, taking 37 percent of the vote against the president's winning total of 53 percent. Buchanan went on to chip away at Bush, winning between 20 percent and 35 percent of the vote in primary after primary. When it was over, Buchanan totaled 22 percent of the vote overall.

He did it on the strength of a solid agenda. Reading Buchanan's Dec. 10, 1991, speech announcing his candidacy, one is struck today by how contemporary it sounds - Buchanan staking out positions on trade, nationalism, interventionism, culture and the economy that seem remarkably current. "We will put America first," Buchanan declared. Besides his obvious talent, Buchanan had other advantages over today's challengers. Perhaps the biggest is that he was the only GOP opponent of the president. The other was that Bush had always had a problem with the more conservative wing of the Republican Party.

"That's where the vacuum was," Buchanan recalled. "It was among conservative Republicans dissatisfied with Bush, who believed Bush had promised certain things, and hadn't delivered, and didn't care about them." That is how Buchanan, a conservative favorite, won 37 percent of the vote in New Hampshire against a president of his own party. But is there an analogous situation today with Trump, not among conservatives, with whom Trump is popular, but with moderate Republicans? Perhaps there is an opportunity for a hypothetical not-Trump candidate. But it seems unlikely that Weld, or Walsh, or Sanford would be that candidate.

The president has serious reasons to worry about losing in the general election. In the RealClearPolitics average of polls, his job approval rating stands at 43 percent, against a 53.9 percent disapproval rating. Even though Trump won in 2016 with a high personal disapproval rating, there's no assurance the states that gave him the election by narrow margins last time - Florida, Pennsylvania, Michigan and Wisconsin - will go for him again next year. But a Trump defeat, should there be one, would be the result of Trump himself, and not his GOP opponents. Separately or as a whole, today's challengers are simply not on the level of the Kennedys, Reagan or Buchanan. Still, some of Trump's opponents hope a primary challenge might cripple Trump. Nothing is impossible, but the fact is, 2020 is not 1992, or 1980, or 1976. Trump might indeed lose, but it won't be at the hands of the retreads who are challenging him in the GOP primaries.

Byron York is chief political correspondent for The Washington Examiner.

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