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 number of votes among straight voters nationwide, exit polls showed. And, a new study argues, Mr. Romney appears to have won a narrow victory among straight voters in the swing states of Ohio and Florida.