Maryland Gov. Martin O'Malley (D) speaks in support of a same-sex marriage bill during a news conference in January. (AP Photo/Patrick Semansky)

With the contribution, the organization said it will have spent $1.6 million to date in Maryland on this year’s legislative battle and the planned referendum on the new law in November.

The increased spending is part of what is expected to be a multimillion-dollar campaign in the Free State, with both sides relying on national organizations for help funding TV advertising and other campaign efforts

Besides Maryland, three other states — Maine, Minnesota and Washington — also have measures related to same-sex marriage on the November ballot.

“This is a tipping-point year in the fight for marriage equality that requires significant investment,” said Chad Griffin, president of the Human Rights Campaign. “We are committed to making sure this is the year that our opponents can no longer claim Americans will not support marriage equality at the ballot box.”

Maryland Gov. Martin O’Malley (D), who signed the state’s new law in March, also has been actively raising money for the referendum, including at an event last month in Chevy Chase that organizers said netted $250,000. The money went to Marylanders for Marriage Equality, the group leading the campaign to uphold the law.

The referendum was triggered by a successful petition drive by the Maryland Marriage Alliance, the lead group fighting the law, which is scheduled to take effect Jan.1, if it is not overturned.

Voters in Washington are also being asked whether to affirm a same-sex marriage law passed by the legislature there — something voters in no other state have ever done.

In Maine, voters will be asked whether to affirmatively allow marriage for gays and lesbians. In Minnesota, voters will consider a constitutional amendment to ban same-sex marriages.