Donald Trump, faced with opposition inside and outside his party, plans to renew the nationalist themes that built his base and amplify his no-holds-barred attacks against Hillary Clinton to try to depress Democratic voter turnout, his advisers said.

Following the release of a tape-recording of his lewd comments about women and several high-profile Republican defections over the weekend, Mr. Trump has effectively given up the conventional wisdom of trying to reach voters far outside his core of support, one high-level Republican...