National polls. It's easy for some congressman, or even some senators, to take a stand against gay marriage since their electorate are local in more conservative districts. Once a candidate's ambitions turn toward the White House their considered electorate suddenly takes on a broader, national position. Issues that got them elected in Kentucky or the 9th district of Louisiana may be the same issues that keep the Presidency out of their reach. Take the Presidency away from a party for a couple of terms and they will marry sheep if they think it will get them back there. To politicians morality is only a tool of the trade.