Maile Pearl Bowlsbey made history in the United States Capitol on Thursday. And she’s only 10 days old.

Cradled in the arms of her mother, Senator Tammy Duckworth, Democrat of Illinois, baby Maile (pronounced My-lee) now has the distinction of being the first infant ever brought onto the Senate floor. Ms. Duckworth brought her daughter to work so that she could be with her baby and vote against the confirmation of a new NASA administrator. (Ms. Duckworth, too, has made history, by becoming the first senator to give birth while in office.)

Tough-minded and staid senators cooed and fussed. “She’s so beautiful,” Senator Chuck Schumer, Democrat of New York, exclaimed. Reporters, looking down from the balcony, responded with a collective “aaawwww.” (“The press is finally interested in something worthwhile,” Mr. Schumer quipped.) Even Senator Mitch McConnell, the ordinarily dour Republican leader from Kentucky, gave a little wave and smiled.

Maile’s arrival was the product of several months of behind-the-scenes negotiation in the hidebound Senate, whose rules until Wednesday barred children from coming onto the Senate floor. A few months after Ms. Duckworth announced she was pregnant, she asked Senator Amy Klobuchar of Minnesota, the senior Democrat on the Senate Rules Committee, to help her engineer a rule change, necessary because senators are required to vote in person.

So it was that on Wednesday, senators voted unanimously that, henceforth, both male and female senators will be permitted to bring infants up to age one into the chamber. In an institution where a fair number of members are in their 80s, this was a monumental change, hardly as simple as it might seem.

First, Ms. Klobuchar said, there were the discussions over whether the Senate floor would turn into some kind of nursery.

“But what if there are 10 babies on the floor of the Senate?” Senator Orrin G. Hatch, 84, Republican of Utah, asked, according to The Associated Press. Replied Ms. Klobuchar: “That would be wonderful and a delight.”

Then came the concerns about whether there would be diaper changes. Or breast-feeding. (Ms. Duckworth, Ms. Klobuchar said, has no intention of doing either while on the Senate floor.)

Some senators wanted an exception only for Ms. Duckworth, an option the two women rejected because they felt it important for lawmakers to send a message to the nation — and to women who might run — that the Senate is a welcoming workplace. Some suggested Ms. Duckworth vote from the Senate cloakroom, which lacks wheelchair access; Ms. Duckworth, a combat veteran who lost both legs while serving in Iraq, uses a wheelchair.

And then there was the baby’s attire: “The baby will not be required to wear pants or a skirt or a tie. The baby will be allowed to wear a baby cap, unlike senators who can’t wear a hat,” Ms. Klobuchar rattled off (Baby Maile wore a pink cap on Thursday.). “Senators are not allowed to wear sneakers or flip flops, but they are allowed to wear orthopedic shoes. The babies don’t have to wear shoes. They could wear baby booties. The baby is also not required to wear a Senate pin, because it would be dangerous.”

Maile’s Capitol debut came at a moment of high drama in the Senate, as lawmakers were split along party lines while voting on the confirmation of Jim Bridenstine to be the NASA administrator. With Vice President Mike Pence on hand to break a possible tie, the Senate extended its customary 15-minute voting window so that Ms. Duckworth could travel from home, as the corridors buzzed with news of her impending arrival. Earlier in the day, she had teased baby-lovers by posting a picture of Maile’s outfit on Twitter.

Maile is the second child born to Ms. Duckworth and her husband, Bryan Bowlsbey, who also have a three-year-old, named Abigail. As her colleagues waited for her arrival, Ms. Klobuchar paced the Senate floor like an expectant father.

Just outside the chamber, Senator Claire McCaskill, Democrat of Missouri, lamented she had a plane to catch: “I’m going to miss a baby, darn it,” she said.

But when the brass elevator doors opened in front of her, out came Ms. Duckworth, her newborn child tucked against her chest, bundled in a blanket. “Here she is!” screamed Ms. McCaskill, who dashed onto the Senate floor, announcing, “She’s coming!”

Ms. Duckworth wheeled in, to so much commotion that the reason she had come — to vote on Mr. Bridenstine — was almost forgotten. She signaled her disapproval, flashing a theatrical thumbs down, with baby Maile, seemingly content, cuddled in her lap.

And echoed the views of women everywhere as she arrived at the Capitol: “It’s about time.”