(Reuters) - After a watershed year for gay marriage in the United States that included ballot victories and a presidential endorsement, advocates have staked out a handful of states where they believe the next round of fights over same-sex unions can be won.

But any victories will not come easily.

Those who defend marriage as a union of one man and one woman plan to use the November 6 defeats as a rallying cry and fundraising tool. The National Organization for Marriage, the leading group opposed to gay marriage, has called on allies to raise $30 million in the coming year - more than double its haul in 2012.