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.

This is not to say that the Democratic coalition is invulnerable. Polls suggest that the party is not faring as well among young or Hispanic voters as in years past. But the inability of Republicans to secure additional gains is a telling indicator of how hardened political sentiments are in the post-Bush, late Obama era. It also shows the limited appeal of Republican candidates to the voters who supported Mr. Obama in 2012 but are dissatisfied with him now. Mr. Obama and his low approval ratings won’t be on the ballot in 2016; the Republican Party and its even lower approval ratings will be.

Above all, the pattern is a reminder of how difficult it will be for Republicans to overcome the demographic and generational changes that have marginalized their traditional coalition in presidential elections. Republicans fare poorly among every large demographic group that is a growing share of the electorate, including Latinos, Asian-Americans and young adults.

Republicans need to compensate for that demographic change by picking up voters who have traditionally supported Democrats in presidential elections. To do so, they will need to broaden their appeal with different candidates and messages, or count on external forces — like a weak national economy, or an emboldened Democratic Party that takes increasingly liberal and unpopular stances.

To a certain extent, the Republicans are already compensating. They remain competitive nationally, despite the large demographic changes, because the Democrats have lost a huge share of support among white Southerners over the last decade. These losses have given the Republicans a decided advantage in the House, and are giving them an opportunity to win the Senate this year. But the Republican gains have done little good in the presidential battleground states, which are largely outside the South, putting Republicans at a slight disadvantage in the Electoral College.