As Senator Scott Brown and Elizabeth Warren prepare for their second debate tonight, a new poll shows Warren leading Brown by 2 percentage points — 46 percent to 44 percent, with 9 percent undecided in the hotly contested race.

The WBUR/Mass Inc. poll of 504 likely voters taken Sept. 26- Sept. 28 is the seventh of nine surveys taken in September to show Warren, a Democrat and Harvard Law School professor, with a small lead over Brown, a Republican. But the results are well within the poll’s margin of sampling error of plus or minus 4.4 percent.

The same polling group had Warren up by 5 percentage points in a poll taken two weeks earlier, from Sept. 15 through Sept. 17.

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Warren’s lead goes up to 4 points in the newest poll from WBUR if the responses of so-called “leaners” are included. These are voters who initially say they are undecided, but express a preference when asked which way they are currently leaning.

The Globe released its own poll Sunday, which showed Warren leading 43-38, with 19 percent of those surveyed saying they remain undecided.

The candidates will debate for the second time tonight, for an hour before a crowd of 5,700 at the University of Massachusetts Lowell’s Tsongas Center. David Gregory, host of the NBC News program “Meet the Press,” will moderate. The debate is co-sponsored by the Boston Herald.

The debate begins at 7 p.m. and will be shown on WHDH-TV (Channel 7), NECN, and CSPAN. Boston.com, bostonglobe.com, and “Political Intelligence” will provide this live blog in the runup to the event and during the candidate back-and-forth.