But those arguments have not moved most congressional Republicans, especially those in the House. The House GOP has essentially barricaded itself against the demographic trends that have helped Democrats win the popular vote in five of the past six presidential elections: 80 percent of House Republicans represent districts in which the white share of the population exceeds the national average. Cantor was one.

Polls consistently show that even most Republican partisans believe that immigrants here illegally should be allowed to stay—and either become citizens or, at least, work openly. But many Republican legislators believe that, as with gun control, those who oppose legalization vote on the issue more consistently than those who support it, especially in the conservative districts they mostly represent. That conviction is certain to be cemented by Cantor's loss to the underfunded Dave Brat, who lashed him for championing "amnesty," despite Cantor's support for only very limited reforms.

As in 2012—when Romney made a crippling commitment to "self-deportation" for those in the country illegally— GOP presidential candidates could be pulled to the right if immigration reform isn't resolved legislatively before the 2016 primaries. Cantor's loss may also prompt Obama to take more aggressive executive action to provide relief for undocumented immigrants. Republican hopefuls will feel enormous pressure to oppose that, as well.

Both of those developments would limit the GOP's ability to improve its 2016 performance among minorities, who have provided Democrats almost exactly four-fifths of their votes in all but one presidential election since 1976. And that would mean the GOP could recapture the White House only if it expands its margins among whites or increases that group's share of the vote by raising turnout.

Neither would be easy. The white share of the vote has decreased in every presidential election since 1980 except one, and minority-population growth virtually ensures its continued decline. Disenchantment with Obama might offer the GOP a somewhat better chance of increasing its margin with whites. Polls show that only about one-fourth of whites or fewer believe they have benefited from either Obama's economic agenda or his health care plan. And the stubbornly slow economic recovery—plus a series of government missteps, including the health care rollout—have moved white voters, in particular, from receptivity toward greater federal activism after George W. Bush's presidency toward a renewed skepticism. "Obama has a taken a majority viewpoint that we need a more aggressive government … and gone 180 degrees in the other direction," says GOP pollster Glen Bolger.

Which returns us to Hillary Clinton. If she runs, the resurfacing doubts about Washington, particularly among whites, would present her with a problem similar to Bill Clinton's in 1992: formulating an agenda that convinces skeptical voters they will benefit from more government activism, rather than less—as Republicans will argue. But even so, it's a stiff bet for Republicans to gamble 2016 on holding Clinton below the 39 percent of whites Obama carried in 2012.