Recently, Massachusetts’ former Republican governor Bill Weld announced he will challenge President Trump in the Republican primaries for the 2020 election. In the 1990s, Weld served two terms as a centrist, socially liberal, fiscally conservative governor of the Bay State. In 2016, he joined Gary Johnson on the Libertarian Party ticket. When making his announcement on CNN, Weld told audiences that he “feared for the republic” should Trump win a second term of office.

Unseating a president from his own party isn’t easy. Weld plans to target the first-in-the-nation New Hampshire primary as the most promising venue for starting a political upset, and to go on to primaries with less restrictive rules about who can participate and where the GOP has suffered setbacks, such as Wisconsin.

Can he succeed? Past primary insurgencies may offer clues.

When have insurgents challenged a sitting president before?

My research on “the politics insurgents make” reveals electoral insurgencies take different forms, with distinct effects on parties and elections. Insurgents and their supporters have taken over parties, launched new parties, attacked governing policies, or pushed policies in new directions. Railing against corruption, monopoly and “politics as usual,” insurgents have pressured parties to adopt new positions, integrate new constituencies or dissolve coalitions.

Third-party challengers — such as Teddy Roosevelt in 1912, George Wallace in 1968 or Ross Perot in 1992 — can act as election spoilers by splitting the major parties’ vote and indirectly helping defeat the incumbent. At other times, insurgents win a major party’s nomination — as happened with George McGovern in 1972 or Donald Trump in 2016 — in an attempt to reshape its program and identity.

Weld is in a different category altogether. Unlike most past insurgents, Weld comes from inside the party to challenge a sitting president for his own renomination. While uncommon historically, such primary challenges against sitting presidents have grown since the 1960s, when party changes led to the primary system we know today. In the old days, when a sitting president pursued a second term, the party’s convention delegates would almost certainly pledge their support and nominate him again. Why mess with success? But today’s primary system offers an opportunity for an insurgent to collect a slim majority of convention delegates.

For instance, Democratic senators Eugene McCarthy and Robert Kennedy both challenged Lyndon Johnson in 1968 before the modern primary system existed. Republican Ronald Reagan narrowly failed when he challenged President Gerald Ford in 1976, but of course returned to win the nomination and presidency in 1980. Democratic senator Ted Kennedy challenged President Jimmy Carter in 1980. And Patrick Buchanan entered the Republican primaries against President George H.W. Bush in 1992.

None of these insurgents stole the nomination from the incumbent president — although Lyndon Johnson did withdraw from the race. But in all four cases the man who did win the nomination — the president, or in 1968, the vice president — lost the election.

Primary challengers don’t cause presidents to lose the general election. But they’re in a good position to change the party when that happens.

Of course, primary challenges do not necessarily cause presidents to lose general elections. It could be that weak presidents with poor favorability ratings, foreign policy crises and economic downturns are likely to be challenged in the first place. But after an incumbent’s defeat in the general election, nomination challengers have often been in a powerful position to reshape the party.

Before Weld, all nomination challengers have come from the parties’ ideological extremes: insurgent Democrats who accuse their presidents of straying from New Deal and Great Society liberalism, or insurgent Republicans attacking their presidents for compromising conservative principles.

Weld is the first to challenge a sitting president from the party’s center

Never before has an incumbent president’s renomination been challenged from the political center. To be sure, Weld has positioned himself as a genuine fiscal conservative in contrast with the Trump administration’s fiscal irresponsibility. But he has also framed his campaign as a protest against “both major parties” and promised to appoint a bipartisan cabinet if elected.

Weld appears not only to be running against Trump, but also against the polarized U.S. party system, of which Trump is the latest and most extreme example. Can that work?

Historically, nomination challenges have come from the two parties’ ideological extreme wings. Such insurgencies have brought together frustrated and marginalized constituencies fighting to steer their parties in different directions. While they haven't unseated a sitting president, they contributed to driving the parties further apart.

All insurgents face daunting odds. Incumbents have major advantages. But Weld’s bid to dump Trump invents a new political style: a moderate insurgency against an extremist establishment. Primary voters are more ideologically extreme than general election voters. While they may respond to Weld’s attack on Trump’s fiscal irresponsibility, they’re likely to reject his other talking points. Polls consistently show that Trump’s hardcore base of support — approximately one-third of the electorate — has been remarkably stable in the approval ratings. They’ll be voting in New Hampshire.

On the other hand, Trump’s small base sets the bar fairly low for a political upset. No previous nomination challenger has ever actually won the New Hampshire primary against an incumbent president. Reagan came the closest in 1976 with 48.6 percent of the vote. But stronger-than-expected second-place finishes in early primaries can catch the media’s attention and encourage the public to support an underdog candidate, even if it’s only a symbolic protest vote.

Can Weld revitalize the Never Trump movement — or has the Republican Party become the party of Trump? We’ll find out.

Adam Hilton (@adhilt) is a visiting lecturer in the politics department at Mount Holyoke College in Massachusetts.