She appeared in an elegant dress in Vogue magazine, and is co-captain of the congressional softball team. Seemingly always working — she has a book out next September — Ms. Gillibrand nonetheless leaves the office promptly at 5 every night to pick up her children from school. If there is a vote at that hour, she has developed a system to signal her aye or nay from a doorway off the Senate floor — where children are not permitted — so she can hold onto her 5-year-old’s hand. “She is ubiquitous,” said Senator Susan Collins, Republican of Maine, “and I mean that as a compliment. I don’t know how she does it.”

Some of Ms. Gillibrand’s Democratic colleagues are less enamored, likening her zeal to that of the Tea Party Republicans who hew to a belief and won’t let it go, ignoring some of the structural protocols of seniority.

“She is unwilling to knuckle under to demands for deference,” said Eugene R. Fidell, who teaches military justice at Yale Law School. “It is very rare that you see a relatively junior member of the Senate staking out a position and sticking by it.”

Ms. Gillibrand’s taste for a fight was presaged in her decision to run for the House in 2006, when she took on Representative John E. Sweeney in a Republican-rich district in upstate New York.

When she approached Howard Wolfson, a Democratic strategist, for help, he told her she could not win. “She told me she was going to run, was going to win, and I would either be the winning consultant or someone else would be,” he said. “I took the race.”

The campaign was one of the nastiest of the cycle, with a spate of negative ads that depicted Ms. Gillibrand as alternately dippy and a war profiteer for investing in war bonds, and ended with the release of a police report that detailed a domestic violence call made from Mr. Sweeney’s home. Ms. Gillibrand won over 50 percent of the vote. “She never wavered, never faltered,” Mr. Wolfson recalled.