As he seeks to revive his embattled candidacy, Donald J. Trump has seized on a new argument to rally his supporters and to explain away a possible defeat in November: that Democrats are preparing to exploit weak voter identification laws to win a “stolen election” through fraudulent voting. The claim has spurred outrage among Democrats and has alarmed some Republicans who worry his tactics will backfire, angering minority voters and threatening the party’s chances in close races down the ballot. Since 2010, Republican governors and Republican-held state legislatures have fought for stricter voter identification laws, which Democrats argue are intended to hinder turnout by the poorest voters, many of them black and Hispanic, who tend to vote Democratic. But Mr. Trump’s language has moved beyond his party’s call for rigid identification requirements and the unfounded claims that polls are “skewed” to predictions of outright theft of the November election. And his warnings have been cast in increasingly urgent and racially suggestive language, hinting that the only legitimate outcome in certain states would be his victory.