Republicans succeeded in one of their top goals this year: They brought more of their party’s voters to the polls than did Democrats. But in the course of energizing their core voters, Republicans in many states lost voters in the political center—both independents and many Republicans who are uneasy with elements of the party’s focus under Mr. Trump. Undercutting the GOP advantage was that independents favored Democrats by 4 points nationally, the survey found, and by a far more substantial 18 points in Pennsylvania, 28 points in Georgia and more than 30 points in Arizona.