Can Bill Weld unseat Trump? Let’s look at the history of challenges to incumbent presidents.

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.