Financial disclosure documents filed Monday show the team has spent $1.4 million in cash and manpower on the effort.
Santa Clara Plays Fair, the group opposed to the $937 million project, in contrast, has raised less than $3,000.
Funding for the stadium would include $114 million in public contributions. Supporters say it would generate tens of millions of dollars for the region. Opponents say it would drain city coffers.
One of those opponents, Deborah Bress, filed a lawsuit this month against Santa Clara seeking to remove or change the stadium measure on the June ballot.
City officials say the suit is without merit.