The proliferation of super PACs and other unrestricted outside groups is further marginalizing women campaign donors, who are already vastly outnumbered by men, according to a report released Tuesday.

Women make up only about 30 percent of political donors overall, a figure that has remained largely unchanged over the last decade, but they gave even less — just under 20 percent — of the money that went to outside groups in the 2012 elections, according to the report, titled “Money in Politics with a Gender Lens.”

“Amongst both general donors and ‘mega donors,’ to super PACs, women continue to be underrepresented and outnumbered by men,” said Kelly Dittmar, an assistant research professor at the Center for American Women and Politics at Rutgers University, which co-authored the report with the Center for Responsive Politics. “As super PACs increase in influence, we find that to be significant.”

Women gave more to outside groups in the 2012 cycle than in the previous election, almost doubling their share of overall contributions. But they remain left largely out of the high-dollar inner circle of donors and groups that increasingly dominates politics. This raises questions about parity and representation, concludes the report, a project of the National Council for Research on Women.

“If money is an expression of political voice, more money may mean a louder voice in debates about public policy priorities and decisions,” states the report, which also cites previous research suggesting “large donors are more likely to be white and male.”

Only a dozen of the top 100 donors to outside groups in 2012 were women, the report notes. The top female donor was physician Miriam Adelson, the wife of casino mogul Sheldon Adelson; she gave more than $42 million to outside groups, all of them conservative, according to the Center for Responsive Politics.

The disparity in female political giving disproportionately harms Republican women, the report found. That’s in part because female donors tend to support women candidates more than male donors do, and most of the top female donors are Democrats. Among the top dozen women outside group donors, only one besides Adelson gave to GOP-friendly groups. The rest supported Democratic spenders. Conservative outside groups vastly outspent their liberal counterparts in 2012, but Republican women candidates did not reap the benefits of that advantage.

“Republican candidates overall have thus far benefited more” from outside spending, said Dittmar, “but Republican women don’t seem to be getting the same boost as the men.”

That’s also because Democrats have a more robust infrastructure to draw in women donors and direct money to women candidates, the report notes, citing the $7.7 million spent by Women Vote!, the super PAC run by the Democratic women’s PAC EMILY’s List. Republican women have set out to even out the scales, setting up a super PAC dubbed Women Lead, and mounting fundraising and policy programs to promote women candidates at all levels.

Women donors in both parties have been overlooked by analysts, according to the report, noting that even Center for Responsive Politics tends to report political giving by couple, listing the contributions of big women donors with those of their spouses, not separately.

Fundraisers have also overlooked women, according to GOP fundraiser Lisa Spies, who helped raise $23 million for GOP presidential nominee Mitt Romney from top female donors in 2012. Spies said many women asked to donate to Romney wondered why they had never been asked before, and that in many households women are the ones writing the checks.

“If you start to ask for women, they will give,” said Spies, who noted that fundraisers who call her house often ask for her husband, GOP lawyer Charles Spies. She is more likely to respond, she added, “if someone calls me and says: ‘Hi, Lisa. This is Governor Scott. I wanted to call you. I’m asking money from you.”

Other findings from the report include: Gender plays less of a role than candidate status and party in determining which politicians super PACs support. Also, women candidates at the state level are more likely to opt into public financing than men, but such systems have done little to increase parity, at least so far. The report also found that, despite some challenges raising early money, women candidates keep pace with or outraise their male opponents in comparable races.