The stakes for Obama are significant, as he tries to preserve Edward M. Kennedy’s long-held Senate seat for Democrats and deflect the sudden and unexpectedly strong candidacy of state Senator Scott Brown, who would become the 41st Republican in the US Senate and give the party the filibuster power it needs to derail the president’s health care over haul.

On a day when the race seemed to grow even more politically charged and vitriolic, Brown responded with a rally of his own at Mechanics Hall in Worcester, which drew nearly 2,000 people. The candidate, until recently a relative unknown, has been drawing increasingly large crowds in this overwhelming Democratic state, with recent polls showing him in a dead heat with Coakley or slightly ahead.

The Worcester event, coined “The People’s Rally,’’ was headlined by a cast of New England celebrities with working-man personas - former Red Sox ace Curt Schilling; Doug Flutie, the former Boston College and NFL quarterback; comedian Lenny Clarke; and John Ratzenberger (Cliff from “Cheers’’) - as Brown sought to cast himself as a regular guy looking to send a message to Washington.

“The voters are doing their own thinking, and the machine politicians don’t quite know how to react,’’ Brown told the crowd. “This Senate seat does not belong to one person, or [one] party. It belongs to you.’’

Both campaigns said they had overflow crowds. Brown’s campaign estimated that 500 people watched a video feed at a nearby hotel ballroom; the Coakley campaign said that as many as 2,500 watched the Obama speech on a live feed across the street.

The Republican - who has made much of the fact that he owns a pickup truck with 200,000 miles on it - rallied the crowd with one of the frequent refrains of his campaign: “I’m Scott Brown, I’m from Wrentham, I drive a truck, and I’m asking for your vote.’’

Obama, in a reference to that Brown campaign line, told the crowd at the Boston rally: “You’ve got to look under the hood.’’

During his 25-minute speech, Obama criticized Brown’s record, saying that the state senator has voted with Republicans 96 percent of the time and that it would be “hard to suggest’’ he would be independent from the Republican agenda.

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