At its peak, in September and again in November, 30 percent of Americans "strongly" backed the proposed changes. With the plan still undergoing modifications, that has dropped to 22 percent, a new low. Substantially more, 39 percent, are "strongly" opposed, a number that's held steadier.
While special elections do not usually draw large crowds, Massachusetts election officials noted relatively high voter turnout across the state despite frigid and snowy weather in some parts of the state.
Brown's candidacy has been credited with energizing Republicans and Tea Party activists, who poured money into the state to help him campaign. A Republican source familiar with Brown's fundraising told ABC News that Brown raised more than $1 million online every day last week.
He ran what many strategists saw as a creative and spirited campaign, carefully crafting himself as a Republican.
In one ad, Brown associates himself with Democrat and former President John F. Kennedy.
Historical footage of JFK stating that money "placed in the hands of consumers and businessmen will have both immediate and permanent benefits" to the economy is followed by Brown, who adds a conservative twist, saying "every dollar released from taxation" will have a similar effect.
Brown could be sworn in as early as Jan. 29.
ABC News' Jake Tapper, Jonathan Karl, Teddy Davis and Huma Khan contributed to this report.