BOSTON — Martha M. Coakley, the Democrat running for Senator Edward M. Kennedy’s seat in Massachusetts, had seemed so certain of winning the special election on Jan. 19 that she barely campaigned last month.

But the dynamic has changed in recent days. ...

And a new poll that showed a competitive race between Ms. Coakley and Mr. Brown has generated buzz on conservative blogs and energized the Brown campaign — though many news organizations dispute its methodology.

Brown is unlikely to win, but if he does, the implications for the health care debate are huge.