Republicans thinking about opposing President Trump in the 2020 primaries are facing the hardest of political choices.

Toppling a sitting president of your own party is a maneuver with the highest degree of difficulty. The most relevant historical model is probably Eugene McCarthy’s race against President Lyndon B. Johnson in 1968, which helped convince the politically wounded Johnson to withdraw. But McCarthy had a clear policy handhold — opposition to an increasingly unpopular war — and appealed to a discontented element within his party.

What are the handholds for a challenger to Trump? Economic conservatives are generally happy with the 2017 tax cut. Social conservatives are generally satisfied with Trump’s judicial nominees (and should be). Foreign policy conservatives are generally not pleased with Trump’s sabotage of alliances, his compulsive personal diplomacy and his abdication of leadership in promoting American values, but the Republican foreign-policy establishment was almost uniformly opposed to Trump the last time around, and it mattered not at all.