US gay rights group Human Rights Campaign has donated $1m to marriage equality efforts in four states.
Campaign groups in Maine, Maryland, Minnesota and Washington will each receive $250,000.
Maine voters will decide whether to legalise gay marriage this year, while voters in Maryland and Washington will decide whether to retain marriage equality laws already passed into law.
In Minnesota, voters are considering whether to back a constitutional ban on gay couples marrying.
HRC president Chad Griffin said: “This is a tipping point year in the fight for marriage equality that requires significant investment. 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.
“Bans on marriage for same-sex couples have sent the devastating message to young people that they cannot grow up to live their dreams and be full and equal citizens. This is the year we will change that.”
He added: “All of these campaigns are winnable but they need resources to educate voters and fight back the lies from groups like the National Organization for Marriage. The country is moving in the direction of equality and a win in any of these states will show that marriage equality is quickly becoming a mainstream, American value.”
According to a June CNN poll, support for marriage equality is at 54 per cent. Polls in Maryland, Maine and Washington suggest voters will back marriage equality, while polls in Minnesota suggest opposition to the proposed constitutional ban.