BOSTON -- When Elizabeth Warren created the Consumer Financial Protection Bureau in Washington two years ago and sought to become its director, she was fiercely opposed by Republican senators who feared she had a visceral loathing of financial institutions and would be a thorn in their sides.
President Obama was so convinced she could not win Senate confirmation that he did not even bother to nominate her.
Now, Ms. Warren, 63, is returning to Washington as a member of the very club that sought to block her and dilute the power of her consumer bureau. She got there by campaigning against the big banks and lobbyists, the millionaires and billionaires who, she said, rigged the system against the middle class.
The question now is how she will approach her job as the newly elected Democratic senator from Massachusetts. How will she interact with those who spurned her? How can she most effectively fulfill the populist promise of her candidacy while serving in an institution that runs on seniority and prefers deference to defiance?
So far, she has sent mixed signals. As she thanked her campaign workers in her victory speech on election night, she said, "You took on the powerful Wall Street banks and special interests, and you let them know you want a senator who'll be out there fighting for the middle class all of the time."
But shortly thereafter, she spoke of compromise and balance and said she had learned the importance of bipartisanship from her Republican opponent, Senator Scott P. Brown.
On Thursday, at her first formal news conference since the election, the normally feisty and loquacious Harvard law professor was about as low-key as she could get without disappearing. She responded to some questions with just a word or two. She would not say what committee assignments she might seek. Even on the subject of the record number of women in the Senate, a response had to be dragged out of her. (Twenty is better, she said, but not good enough.) Ms. Warren is a quick study, and perhaps she had already learned that campaigning is different from legislating. After the news conference, she told a smaller group of reporters that as a senator she needed to be discreet.
"Listen," she said, "all I can say is I was a lot more discreet as a candidate than I was in real life." She then turned to an aide. "Can I say that?" she asked. "Maybe it's indiscreet to talk about discretion."
Bankers and Wall Street types might be surprised to hear that during the campaign Ms. Warren held herself in check.
"Wall Street C.E.O.'s -- the same ones who wrecked our economy and destroyed millions of jobs -- still strut around Congress, no shame, demanding favors and acting like we should thank them," Ms. Warren declared in her prime-time speech at the Democratic National Convention.
That was the Elizabeth Warren who earned the adoration of millions of people across the country, the one who spoke truth to power and was not worried about sounding indiscreet. And it is what they expect of her now that she has a powerful new platform in Washington.
"Elizabeth Warren is a doer," said Neil Barofsky, who, as the special inspector general for oversight of the bank bailouts, worked with Ms. Warren and wrote about her in his book, "Bailout." He added, "I think she would suffocate if she went down to the Senate and kept her head down and played nice and made compromises."
Hillary Rodham Clinton is often cited as the model of a national celebrity who came to the Senate, kept her head down, worked hard and built relationships across the aisle.
"Hard edges don't work very well in the Senate," Ross K. Baker, a political scientist at Rutgers University and student of the Senate, said of Ms. Warren. "Her base regards her, quite justifiably, as a great heroine. But I suspect that MoveOn.org will have unrealistic expectations."
Ms. Warren, speaking last year to New York magazine, rejected the Clinton model.
"If the notion on this is we're going to elect somebody to the United States Senate so they can be the 100th least senior person in there and be polite, and somewhere in their fourth or fifth year do some bipartisan bill that nobody cares about, don't vote for me," she said.
Ms. Warren has also written critically of Mrs. Clinton for compromising on a bankruptcy bill that was supported by the financial industry. When she was the first lady, Mrs. Clinton opposed the bill. But as a senator from New York -- when she needed support from Wall Street, a hometown industry -- she voted for it.
"She couldn't afford such a principled position," Ms. Warren wrote. "Campaigns cost money, and that money wasn't coming from families in financial trouble."
Ms. Warren is not likely to face that particular dilemma. The financial services sector gave Mr. Brown more than $5.5 million to defeat her. The U.S. Chamber of Commerce said that "no other candidate in 2012 represents a greater threat to free enterprise than Professor Warren."
But Ms. Warren raised a stunning $39 million, the most of any Senate candidate this year, proving that it was possible to run against the big banks without Wall Street money and still win.
"She doesn't need their money," Mr. Barofsky said. "That gives her freedom and independence."
Wall Street is bracing for Ms. Warren to stay true to form.
Christopher Whalen, a senior managing director of Tangent Capital Partners, said on Fox News after the election, "Liz Warren is only one of 100 senators, thank God. She's kind of an angry Calvinist."
Ms. Warren's adversaries are said to be trying to keep her off the banking committee, where she could push for more regulation, while her admirers want her to be on it.
"Her strategy will depend on what happens," said Simon Johnson, a professor at the M.I.T. Sloan School of Management. "If she doesn't get on the banking committee, then she'll take a more outspoken approach." He said that Democrats as a whole had not followed through on several issues of financial reform, so "it matters a great deal" where Ms. Warren is assigned.
"Not putting her on banking would make the Democratic Party look like a creature of Wall Street, which, by the way, it is," Professor Johnson said. "But they don't like to be too explicit about it."
Senator Charles E. Schumer, the New York Democrat who helped recruit Ms. Warren for the Senate race, sought to deflect criticism that he was doing Wall Street's bidding and trying to keep her off the banking panel, saying in an interview that he would help her get on whatever committee she wanted. He predicted she would learn to navigate the Senate because she knows how to adapt, just as she adapted during her campaign to become a better candidate.
"She has a national voice before coming to the Senate, but she will be very respectful of her colleagues," Mr. Schumer said. "She can be a strong voice and, at the same time, be a team player."
This article originally appeared in The New York Times.