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?