SAN FRANCISCO—The campaigns for and against a November ballot measure that would outlaw same-sex marriage in California are picking up financial steam.

The sponsors of Proposition 8 announced Aug. 19 that they have received their largest campaign contribution to date: A $1-million donation from the Knights of Columbus

The money comes on top of $250,000 the New Haven, Conn.-based Catholic fraternal organization gave in January to help qualify the measure for the November election.

Last month, the founder of the WordPerfect software company, Bruce Bastian, pledged $1 million to help defeat the initiative, which would amend the state constitution to limit marriage to a man and a woman.

Bastian’s gift is the largest single contribution received so far by the amendment’s opponents.