With four days before the Democratic gubernatorial primary, state Treasurer Shannon O'Brien had a strong lead over her three rivals, a new poll shows.

The Boston Globe/WBZ-TV poll of likely Democratic voters also indicated strength for state Senate President Thomas Birmingham, who lagged in earlier polls of the broader voting population.

O'Brien was favored by 31 percent, followed by Birmingham and former Labor Secretary Robert Reich at 22 percent. Former state Sen. Warren Tolman had 13 percent. Nine percent said they were undecided.

Only 30 percent of O'Brien's supporters described themselves as "firmly committed.'' By contrast, 63 percent of Birmingham's supporters said they were "firmly committed.'' Fifty-five percent of Reich supporters and 53 percent of Tolman's supporters said they would definitely vote for their candidate.

The poll of 400 voters, conducted Sept. 9, had an error margin of 5 percentage points.

The poll also indicated that attempts by O'Brien's rivals to link her performance as treasurer to the dropping value of the state pension fund weren't working. Only 15 percent blamed her, while 60 percent blamed the stock market downturn.