The topic is believed to be behind the very high voter turnout in Missouri's primary election on Tuesday. Seventy percent of Show Me State voters approved a change to the state constitution, allowing it to define marriage as a union between a man and a woman only.
The turnout is the highest for a primary vote in a quarter century.
That has political analysts suggesting that with a race as tight as the one between Bush and Massachusetts Sen. John Kerry (search), any small edge could be a big deal.
"In an election that looks to be as close as this one, you can say any issue swayed the thing," said Fox News contributor and Almanac of American Politics author Michael Barone.
Missouri becomes the fifth state to have changed its state constitution. Another 10 states are set to vote on similar state constitutional changes. Ohio could be the eleventh. Petitions were turned in on Tuesday to put a marriage definition on the ballot.
If the constitutional amendment (search) does make it onto the ballot, even those fighting for gay marriage in Ohio concede that the issue will help Bush with turnout.
Click in the box near the top of the story to watch a report by FOX News' Steve Brown.