WASHINGTON — An hour before her colleagues gathered for their first vote of a new Congress, Senator Kelly Ayotte slipped into an empty Senate chamber to savor the grandeur of her legislative home. As Ms. Ayotte, a freshman Republican from New Hampshire, sat down at the wooden desk where generations of lawmakers from her state had cast their votes, a doorman marched toward her with purpose.

The desks, he sternly told her, were for senators only.

Ms. Ayotte’s induction that January day in 2011 into the most rarefied ranks of the nation’s political class — female senators — had begun. “The desk thing really stuck with me,” Ms. Ayotte said. “There still just aren’t that many of us.”

In the 90 years since Rebecca Felton of Georgia became the first woman in the United States Senate — sworn in for a mere 24 hours — women remain an anomaly in the upper chamber. But with 20 female senators now in office, an all-time high, women have morphed from the curiosity they were for much of the 20th century into an important new force on key committees and legislation.

A record nine women now lead committees, including some of the most powerful ones. For the first time there is a woman — Senator Barbara A. Mikulski, a Maryland Democrat — in charge of the Senate Appropriations Committee, which dispenses billions of dollars annually throughout the government and has long been particularly dominated by men. Senator Patty Murray, a Washington Democrat, is the first chairwoman of the Budget Committee and is charged with shaping the Democratic strategy in the fiscal battle dominating Capitol Hill.

One of the biggest bills to pass the Senate last year was farm legislation led by Senator Debbie Stabenow, a Michigan Democrat who presides over the agriculture committee. Senator Barbara Boxer, the California Democrat who is chairwoman of the Environment and Public Works Committee, shepherded the highway bill.

“We are growing in number,” said Senator Amy Klobuchar, Democrat of Minnesota. “But more importantly, we are growing in our power. When I first started just six years ago, it was unusual to have a woman managing a bill.”

Just as important, even male senators say, is the potential the women hold for changing the tenor of the Senate and pushing for compromise in the highly partisan atmosphere on Capitol Hill.

“I don’t want to generalize, because this isn’t true of all of them, but they tend to be interested in finding common ground,” said Senator Rob Portman, Republican of Ohio. “So I think it’s going to have, and is having, a positive impact on the Senate.”

While partisan division is the central characteristic of the modern Congress, women have begun to crack away at the gridlock by forming coalitions that have surprised leaders of both parties. Senator Kirsten E. Gillibrand, Democrat of New York, and Senator Susan Collins, Republican of Maine, led the repeal in the Senate of “don’t ask, don’t tell” in 2010, allowing gay men and women to serve openly in the military.

This year, all four of the female Senate Republicans split with their party and voted with Senate Democrats to reauthorize the Violence Against Women Act, which had lapsed during the last Congress.

But politics remain just beneath the surface. In a recent roundtable involving the women and ABC’s Diane Sawyer, Senator Elizabeth Warren, Democrat of Massachusetts, sharply disagreed with Ms. Collins on the efforts the women should devote to reproductive rights. After Ms. Collins described reproductive issues as “settled law” and said “I don’t know why we would want to keep bringing those issues up,” Ms. Warren shot back that “I don’t think they are entirely settled” and that “we better speak out” when there are attempts, for example, to limit health insurance coverage for birth control.

“Obviously there is so much more partisanship in recent years, and that has a spillover,” said Olympia J. Snowe, the moderate Republican former senator from Maine who left the Senate last year, dejected by the political polarization on Capitol Hill and what she described as her growing inability to reach across the aisle. But she called the 20 women “a critical mass” that she expected could “largely effect change together.”

On a practical level, the women’s growing ranks have overtaken the physical facilities available to them. There are, at present, a mere two women’s bathroom stalls near the Senate floor, which often means long lines. But the senators have learned to use the situation to their advantage: Ms. Stabenow said recently that she and Senator Heidi Heitkamp, a North Dakota Democrat, spent their time in the bathroom line strategizing over how they might get a new farm bill passed.

“It’s a good problem to have,” Ms. Stabenow said. “We have enough of us now that we can negotiate in the ladies’ room.”

Off hours, the Senate women have a bipartisan dinner together once a month, a ritual organized by Ms. Mikulski, which they say creates personal bonds and helps them work together on policy. At a recent dinner at the Capitol Hill home of Senator Mary L. Landrieu, Democrat of Louisiana, the women nibbled on bread pudding for dessert while they talked about children and, Ms. Landrieu said, “about how our siblings keep us in our places.” She added, “It’s just so nice to be able to relax and put your guard down.”

The dinners “are the only reason I’ve been successful,” Ms. Gillibrand said, recounting how Senator Lisa Murkowski, Republican of Alaska, helped her write the bill that provided health care to the first responders to the attacks of Sept. 11, 2001. “In the Senate you have to start with a bipartisan core to get things done, and that core is often formed with the women,” Ms. Gillibrand said.

A wide array of scholarship supports the view that women tend to cross party lines to build legislative consensus more frequently than men. In a recent article in the American Journal of Political Science, “When Are Women More Effective Lawmakers Than Men?.” the authors found, for example, that the dynamic tends to favor women in the minority party. In particular, the study found “while men may choose to obstruct and delay, women continue to strive to build coalitions and bring about new policies.”

The study found that over the course of 30 years in the House, minority party women were better able to keep their sponsored bills alive in later stages than minority party men, largely by keeping the focus on the underlying policy goal over politics.

Women have also focused on legislation that men do not typically consider, like financial security for women, an issue championed by Senator Kay Bailey Hutchison, the Texas Republican who retired in January. The seven women now serving on the Senate Armed Services Committee have begun to push their priorities, including efforts to prevent and adjudicate sexual assault in the military as well as to open combat jobs to women.

The Senate women have frequently come together on child care and family health legislative issues. During the reauthorization of a bill to finance the military last year, Ms. Gillibrand went to the floor with an amendment to expand coverage for beneficiaries’ children who have autism and other developmental disabilities, financed by a $45 million, one-year payment out of an existing military budget line.

Senator John McCain, Republican of Arizona, and Senator Tom Coburn, Republican of Oklahoma, went to the floor to argue vociferously against the cost of the measure, but it passed, with every female senator voting in the affirmative. “I supported it,” said Ms. Ayotte, one of the most conservative members of the Senate, who has little in common politically with Ms. Gillibrand.

The fragile road of the Senate sisterhood, however, has been laboriously paved one brick at a time. Of the 44 women who have served in the United States Senate, many began as short-term appointees who filled out a dead husband’s term, and often did so as the only women in the chamber.

After Mrs. Felton’s brief sojourn through the Senate in 1922, Hattie Caraway, an Arkansas Democrat, became the first woman to truly serve. Appointed to the Senate in 1931 after the sudden death of her husband, Ms. Caraway shocked her state and party by running in the general election the following year. “She was expected to go back to be the widow from Arkansas,” said Betty K. Koed, the associate historian for the Senate. She won, and kept her seat until 1945.

Next came Margaret Chase Smith, a Maine Republican who served nearly five terms in the House before winning her Senate seat in 1948, a spot she held for 24 years. The occasional woman continued to get elected, including Ms. Mikulski, who arrived in 1987 and in 2012 became the longest-serving woman in Congressional history. (She has served 26 years in the Senate and 10 in the House.)

In 1992, known as “The Year of the Woman,” the election increased the number of women in the Senate to seven from two.

Dianne Feinstein, the California Democrat who is now chairwoman of the Senate Intelligence Committee, was elected that year, as was another California Democrat, Barbara Boxer.

The male senators vacillated between relief — the chamber was crawling out from its antediluvian state — and fear. “When we first got here they were like, ‘What are we going to do with all these women?’” Ms. Murray recalled. “There was a huge fear, though it was never stated: ‘Would these women be bomb throwers or legislators?’”

They had come to work, although it took some convincing. “When I started out it was an absolute disadvantage to be a woman,” Ms. Boxer said. “You had to prove you understood numbers, that you understood history.”

Some were also visibly balancing work and home, although today Ms. Gillibrand and Ms. Ayotte are the only female members with small children.

While Senator Caraway once noted that a run in her stocking “would make headlines,” women today are more likely to draw attention for their legislation. But Ms. Klobuchar remembers that after six months in the Senate, a male senator — she will not say who — stepped into a “Senators Only” elevator, took a look at her and asked what she was doing there.

After the aide with Ms. Klobuchar told the man that Ms. Klobuchar was, in fact, a senator, Ms. Klobuchar looked her male counterpart straight in the eye and said, “But who are you?”