A recent CNN poll showed, for instance, that 51 percent of Americans now prefer an emphasis on border security and deportation versus 45 percent who prefer creating a path to legal status for illegal immigrants — a reversal from the 41 percent to 54 percent margin observed earlier this year. Polling also suggests that opponents of creating a path to legal status feel more strongly about the issue than supporters.

Mr. Obama might hope to mobilize the Democratic base or boost Hispanic turnout, but midterm electorates are skewed toward an older, whiter group of voters who are likely to view a legalization plan less favorably. By contrast, the Hispanic population votes at lower rates and is likely to make up a small share of the electorate in this year’s most competitive House and Senate races (though their role in 2016 could be more significant).

It’s easy to overstate the effects of policy on electoral outcomes, but there is a recent worst-case example: the passage of the Affordable Care Act.