When former Republican congressman Joe Walsh announced last Sunday that he was running against Trump for the party’s nomination for president in 2020, the instant verdict was overwhelming: Walsh does not have a chance in hell. “Mr. Walsh stands virtually no chance of wresting the Republican presidential nomination away from Mr. Trump,” the New York Times wrote. And the Times is correct.

But Walsh’s run could still herald the end of Trump’s presidency.

Since the advent in the 1960s of the modern system of party primaries for presidential nominations, no incumbent president seeking re-election has been successful if challenged in the primaries.

There are no Newtonian rules of politics as there are in physics, but there are certain axioms that have proved true over history. And among the most important is this: Going back to Franklin D. Roosevelt, but really since the advent in the 1960s of the modern system of party primaries for presidential nominations, no incumbent president seeking re-election has been successful if challenged in the primaries.

In 1968, Lyndon Johnson withdrew from the race following challenges from Senators Gene McCarthy and Robert Kennedy. Johnson’s words from the Oval Office on March 31, 1968 remain some of the most haunting ever uttered in a presidential address: “Accordingly, I shall not seek, and I will not accept, the nomination of my party for another term as your president.” The nation was irrevocably changed that night.

In 1976, President Gerald Ford, who assumed office in 1974 following Richard Nixon’s resignation over Watergate, fought off California Governor Ronald Reagan – and lost to Jimmy Carter. In 1980, Carter beat Senator Ted Kennedy to win the re-nomination, and lost to Reagan. In 1992, President George H. W. Bush was challenged by conservative Pat Buchanan – and lost to Arkansas Governor Bill Clinton.

Reagan, Clinton, George W. Bush and Barack Obama never had an opponent to re-nomination and each served two terms.

Aside from coming after Trump from the rigid hard-right of the conservative movement – a part of Trump‘s base – and with an air of moralism against Trump’s personality and style, why will Walsh inflict damage on Trump? Walsh will get measurable media attention. He will challenge Trump to debates during the primaries – and force Trump to publicly ignore him. While Trump has 85 per cent support in the Republican Party, any primary showing where Walsh and the other rebellious contender, former liberal Governor William Weld of Massachusetts, might together take out 25 per cent of a given state’s Republican primary contest (think New Hampshire) will be noteworthy. In other words, Walsh will be a noisy nuisance who will get airtime, and Trump will have to pay attention at least to Walsh and will have to devote resources against him.

All these incursions by Walsh mean that Trump will have to spend some of his political capital and therefore make Trump weaker going into the general election than would be the case if he was truly unopposed for re-nomination.

While this is not an iron law of politics, there is some truth in a half century of testing in the field.

Walsh could well number Trump’s remining days in office.