The evidence is vast. Republicans have frittered away winnable races in Alabama, New Hampshire and elsewhere over the past decade by nominating extremist candidates, while Senator Susan Collins of Maine, a moderate Republican, is the only sitting senator who represents a state that reliably votes the other way in presidential elections. On the Democratic side, there are no progressives in the mold of Alexandria Ocasio-Cortez or Bernie Sanders who represent a swing district or state. Instead, the Democrats who win tough races work hard to signal to voters that they are less progressive than their party.