Even if the Republicans win these states, which would all but ensure Senate control, it will probably be mostly because of low midterm turnout among Democratic-leaning young and nonwhite voters. The implication would be that Republican Senate candidates did not win many voters who supported President Obama in 2012. And it would suggest that Republicans have made little progress in attracting voters they would need to take back the White House.

If there were a time when the Republicans ought to be making inroads into the Obama coalition, this should be it. The economy remains mediocre in many respects; there is turmoil in much of the world; and the American public is decidedly downbeat about the state of the country under Mr. Obama. His approval ratings have sagged into the low 40s. A significant proportion of Democratic-leaning voters say they disapprove of his performance.
Historically, presidential ratings like these have permitted the party that does not hold the White House to make substantial gains. This year, however, Democratic Senate candidates in the battleground states have largely reassembled the coalition that supported Mr. Obama two years ago. Democratic candidates would probably win Colorado, North Carolina, Iowa and Georgia — along with control of the Senate — if those who vote were as young, diverse and Democratic as they were in 2012 or will be in 2016.