DENVER — It wasn’t all that long ago that Republicans used gay marriage as a tool to drive Election Day turnout. But as public opinion on the issue has turned and courts strike down same-sex marriage bans, gay rights is evolving into a wedge issue for Democrats to wield.

Consider Pennsylvania, where Democrats have lambasted Republican Gov. Tom Corbett for comparing gay marriage to incest. Facing a tough re-election campaign, Corbett decided this week not to appeal a federal court ruling striking down the state’s ban of gay marriage.