While President Obama’s lopsided support among Latino and other minority voters has been a focus of postelection analysis, the overwhelming support he received from another growing demographic group — Americans who identify as gay, lesbian or bisexual — has received much less attention.

But the backing Mr. Obama received from gay voters also has a claim on having been decisive. Mitt Romney and Mr. Obama won roughly an equal share of votes among straight voters nationwide, exit polls showed. And, a study argues, Mr. Romney appears to have won a narrow victory among straight voters in the swing states of Ohio and Florida.

Mr. Obama’s more than three-to-one edge in exit polls among the 5 percent of voters who identified themselves as gay, lesbian or bisexual was more than enough to give him the ultimate advantage, according to the study, by Gary J. Gates of the Williams Institute at the U.C.L.A. School of Law, in conjunction with Gallup. The results are consistent with earlier research on the number and political beliefs of gay voters.

Democrats have been winning big over Republicans among gay voters, African-Americans, Latinos, Asian-Americans and Jews. Some of the groups are relatively small, but together they make up about one-third of the electorate, forcing Republicans to capture much of the remaining two-thirds to win elections. By comparison, white evangelical voters, who vote overwhelmingly for Republicans, make up about one-fourth of the electorate, and their numbers are not growing as rapidly.