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.)