President Trump shakes hands with Senate Majority Leader Mitch McConnell before the start of a meeting with House and Senate leadership at the White House on June 6. Also in the room are from left, Vice President Pence, Senate Majority Whip John Cornyn, House Majority Leader Kevin McCarthy, House Speaker Paul D. Ryan and senior adviser to the president Jared Kushner. (AP Photo/Pablo Martinez Monsivais)

The all-male group of Republican senators working to craft a compromise health-care bill has created a public relations problem for the GOP. With no women among the 13 lawmakers chosen by the leadership, Senate Majority Leader Mitch McConnell (R-Ky.) has been criticized for a lack of gender diversity in a group that is dealing with such issues as whether insurance plans must cover maternity care and whether women can be charged more for insurance.

This is not the first time Republicans have been criticized for crafting policy without any women in the room. But the absence of GOP women in high-level policy talks and public photo-ops reflects a larger problem: There are very few Republican women in Congress, so when Republicans control the majority, not many women have access to power. Right now, just 26 of the 104 women in Congress — 25 percent — are Republicans.

There are a variety of explanations for this disparity. Scholars have pointed to the fact that since the feminist movement the Democratic Party has come to be seen as the party of women’s rights. Women’s groups associated with the party — from NOW to Emily’s List — prioritize electing women. Other scholars point to the political pipeline, where more of the women in the jobs that lead to a run for office are Democrats. Democratic women in state legislative office, a common steppingstone to Congress, are also a better ideological fit for their party’s congressional delegation than are Republican women, who are more likely to be moderates.

Here’s how we followed the money

But our recent research suggests another reason: money. In the Democratic Party, female donors disproportionately make it a priority to boost female candidates, giving to Democratic women running for office. That’s not so for Republicans, where both male and female donors make ideology a priority and pay no attention to candidates’ gender. Thus, no group of Republican donors is particularly committed to electing women.

To understand the finance networks of male and female candidates in both parties, we built a unique data set of all candidates who ran in a primary or general election for the House of Representatives in 2010 and 2012. And we combined that data set with Adam Bonica’s measures of candidate ideology that are generated from a candidate’s mix of campaign donations.

We then examined the total amount of itemized individual donations these candidates received from male and female donors. While Super PACs and mega donors currently receive a large amount of attention, most House races are not competitive. More than 90 percent of incumbents who run are reelected.  In 2010, half the money raised by House challengers — those running against incumbents — came from itemized individual donations of $200 to $2,400 in 2010 and $2,500 in 2012.

In both parties, donors tend to prefer candidates who fit clearly with their party’s dominant ideology — liberals for Democrats and conservatives for Republicans. That’s one reason it’s hard for moderates in either party to get elected.

Beyond ideology, gender matters to Democratic donors. We find that female Democratic donors are particularly likely to give to Democratic women running for office. Male Democratic donors are more likely to give to male candidates. Since the vast majority of donors are men, this could make it challenging for Democratic women to raise money.

But Democratic women benefit from the fact that, as a group, female Democratic donors are especially committed to supporting women. One consequence is that female Democratic donors are willing to give to female candidates regardless of whether they are incumbents or challengers. Even at the presidential level, Hillary Clinton, the first female major-party nominee, was also the first presidential candidate to raise more of her individual donations from women. In 2016, 52 percent of Clinton’s individual donors were women, compared with 44 percent of Barack Obama’s individual donors in 2012.

By contrast, male Democratic donors tend to follow conventional patterns, directing more of their money to incumbents, committee chairs and candidates in competitive races who will expand the number of seats held by the party.

As a result, female Democratic candidates are able to raise as much money as their male counterparts, largely because women are disproportionately likely to give to their campaigns both in primary races where they are competing against other Democrats and in general elections.

That’s not so among Republicans. Gender appears to make no difference to GOP donors, male or female, in deciding where to give money, as neither are more likely to give to male or female candidates. Rather, ideology predominates. In fact, female Republican donors support even more conservative candidates than male Republican donors.

Consequently, a Republican woman seeking office must be a committed conservative. She must compete for the same pool of donors as her male Republican counterparts, and cannot count on a committed core of Republican women to boost her support. As a result, Republican men raise 1.2 times as much as Republican women from itemized individual donations of $200 or more: $737,729 for Republican men, on average, compared with $613,480 for Republican women.

If current trends in donor priorities persist, Republicans face an uphill climb in their efforts to elect more women. And that’s about to get worse. Moving into the 2018 election, Democrats benefit from the fact that the president’s party usually loses seats in midterm election years. This year Democrats are seeing an increasing number of liberal women who, angered by Hillary Clinton’s loss and Donald Trump’s treatment of women, are motivated to run for office. In April, Emily’s list, a group focused on electing Democratic women who support abortion rights, reported that since the election it has heard from 11,000 women who are thinking about running for office, a dramatic increase from the 900 who contacted the group in 2016. The disparity between the parties in women’s representation in Congress is likely to expand — unless Republicans decide to more proactively recruit and fund female candidates.

Michele L. Swers is a professor of American government at Georgetown University.

Danielle M. Thomsen is an assistant professor in the Maxwell School of Citizenship and Public Affairs at Syracuse University.