The cultural and ideological fissures opening in the party could take a generation to patch, according to Republican leaders, historians and strategists — and many are convinced that Mr. Trump will guarantee Democrats another four years in the White House. “Nominating Donald Trump would be the best gift the Republican Party could give to Hillary Clinton,” Bobby Jindal, the former Louisiana governor, said in an interview on Tuesday.

Democrats are now poised to exploit a fortuitous intersection of forces: an improving economy with low unemployment; a Democratic president with a nearly 50 percent approval rating; a Supreme Court battle in which Republicans are energizing liberal voters with vows of obstruction; and now, what is likely to be a relatively smooth nomination process that will give Mrs. Clinton a chance to bring together the party’s disparate strands.

Of course, Mrs. Clinton, should she prevail in the primary campaign, has plenty of repair work left to do: wooing the thousands of liberal supporters whose feelings of alienation with the Democratic establishment drew them to Senator Bernie Sanders of Vermont. She began that effort on Tuesday night, in a victory speech that focused heavily on the Sanders campaign theme of economic justice. And there is little indication, so far, that these voters will spurn Mrs. Clinton for a Republican.

And Mrs. Clinton needs to navigate a series of potentially damning investigations into her use of a private email server that have raised enduring questions about her judgment and management. Those inquiries have introduced a level of unpredictability that her campaign can do little to control.

But officials in both parties acknowledge that Democrats are now better positioned to capture the presidency in November.“The Democrats are having a loud squabble, but the party is broadly unified behind certain themes,” said David Axelrod, the Democratic strategist. “The Republicans are engaged in a full-out civil war, fundamentally riven by mistrust, and it is very hard to see how they put the pieces back together once this fight is done.”