BOSTON — Elizabeth Warren announced her candidacy for the United States Senate here on Wednesday and plunged into street-level politics, greeting commuters at a train stop at the outset of what could be a marquee Senate race of 2012.

Ms. Warren, a Harvard professor and consumer advocate whom Republicans have already branded an outsider — she grew up in Oklahoma — pumped hands, patted shoulders and even ducked into a revered Massachusetts institution, Dunkin’ Donuts, to ask for support.

She then moved on to Worcester, Springfield and other Democratic strongholds for more of the same, perhaps eager to prove she could connect with voters as adeptly as Senator Scott P. Brown, the well-liked Republican incumbent whose seat has become a prime target for national Democrats.

Ms. Warren, 62, is far from being the Democratic nominee. She must first compete in a crowded primary that will not take place until next September. And while many national Democrats believe she has the best chance of beating Mr. Brown, some in Massachusetts, including Mayor Thomas Menino of Boston, have questioned whether voters will take to Ms. Warren as a candidate.