Four Republican state senators crossed party lines in June to vote in favor of New York's historic same-sex marriage bill -- and now a group that opposes same-sex marriage has them in its crosshairs.

The National Organization for Marriage, which was one of the loudest voices against the bill while the State Senate was debating it, has begun an all-out campaign to defeat the four senators when they come up for re-election. Recently, the group purchased billboards in the senators' districts, which read, for example, Roy McDonald: You're Fired!!!

The four senators are McDonald, who represents the Albany area; James Alesi, who represents Monroe County in upstate New York; Mark Grisanti, who represents Buffalo; and Stephen Saland, who represents Dutchess County in the Hudson Valley region. It took a lot of urging from Gov. Andrew Cuomo to get the four of them to vote to legalize same-sex marriage, but once they agreed, they stuck by their decisions and have continued to do so.

People need to get over it, because the simple fact is that everyone should enjoy the same rights, Doug Curella, a spokesman for Grisanti, said to The Daily News.

They are entitled to their opinion, Mike Veitch, a spokesman for McDonald, told The Daily News. It is a free country.

The billboards are the latest and most visible move in an ongoing tussle between the National Organization for Marriage and several gay-rights groups, such as the Human Rights Campaign, that have pledged to support the senators who voted for the bill. The senators can also count on the considerable political clout of Cuomo, who is very popular throughout the state. He, too, promised to do everything in his power to get them re-elected in exchange for their support.

I can help you, Cuomo told the senators while he was lobbying them to vote for the bill, according to The New York Times. I am more of an asset than the vote will be a liability.

The goal of the National Organization for Marriage is not just to get the four senators voted out of office. Ultimately, they want to overturn the bill that legalized same-sex marriage and hold a statewide referendum to define marriage as between a man and a woman.