“The GOP may or may not face danger at the ballot box in 2018 and 2020, but it is doomed in the long run if no Republican stands for the principles that the party has for so long said it defends: governmental restraint and individual liberty. Ultimately, the best way for the GOP to change course — to repudiate Trump and his chaos-inducing, nativist, racist approach — isn’t rocket science: Some Republican must take on the president. And such a challenge would still have an effect, even if Trump won the nomination.

“(S)ince 1968, serious primary challengers to incumbent presidents have been one of the more effective vehicles to push a party to change its ideas, rethink its core constituencies and remake its platform. Eugene McCarthy, Ronald Reagan, Edward Kennedy and Pat Buchanan helped sink the incumbent’s reelection chances and alter their parties’ direction over the long term. They endured ridicule from onetime allies because they had the gall to run against sitting presidents, but their campaigns kept alive within their parties alternate traditions and dissenting views, some of which became dominant over time.”
You can read Dallek’s column, “Why the GOP needs someone — anyone — to challenge Trump in 2020,” at this link.