MANY REPUBLICANS in Congress oppose immigration reform for fear of it creating millions of new Democratic voters and putting the White House forever beyond the GOP’s electoral reach. “This is President Obama’s number one political agenda item because he knows we will never again have a Republican president, ever, if amnesty goes into effect,” Rep. Michele Bachmann (R-Minn.) told World Net Daily, an online publication, in June.

That conviction helps explain the party’s opposition to a path to citizenship for undocumented immigrants. But there are at least two reasons to doubt the Republican assumption. One is that any plan with a chance of enactment will contain a very long timeline for naturalizing the nation’s 11 million illegal immigrants. Under the Senate bill passed in June, immigrants would have to wait 13 years to become citizens; under some House proposals, the wait would be even longer. It is folly to predict how the nation, let alone particular voting blocs, might tilt in the 2028 or 2032 presidential elections.

The second problem is the numbers themselves. Even if all 11 million of these people had magically been made eligible for citizenship — and, for those over 18, the vote — in time for last year’s presidential election, there is no evidence they would have had a major impact on the outcome or on Mr. Obama’s margin of victory in the electoral college.

One of the most careful studies of that theoretical impact was performed by Carson Bruno, a researcher at the conservative Hoover Institution at Stanford University. Mr. Bruno’s analysis, “The Electoral Consequences of Granting Citizenship to Undocumented Immigrants,” suggested that large-scale naturalization in advance of the 2012 elections would have added less than 1 percentage point to Mr. Obama’s margin of victory.

Even using the most pro-Democratic assumptions — exceptionally high rates of naturalization, voter turnout and pro-Obama sentiment among immigrants newly eligible to vote — Mr. Bruno concluded that the president’s 2012 margin of victory would have increased by about 1.6 million votes. That would have padded his 2.5-point victory over Mitt Romney (50.5 percent to 48 percent) only slightly. In the electoral college, where Mr. Obama won overwhelmingly, there would have been no change, with the possible exception of Mr. Romney’s win in North Carolina, which might have moved closer to a tie.

Fear of losing electoral ground is certainly not the only factor driving Republican opposition to immigration reform. Many Republicans believe that any form of amnesty is unfair to would-be immigrants who legally entered the country. Some think legalizing the status of undocumented immigrants would sap the economy, or drive down wages for native-born workers. In some cases, old-fashioned racism may also be a factor.

Politicians being politicians, though, plenty of Republicans are simply weighing the political risks. Fair enough. But in doing so, they should make a clear-eyed assessment.