Outside interest groups are springing up on a daily basis to dump huge sums of money into the US Senate race, funding negative television ads that seek to tip the race in its closing hours.

Massachusetts has become ground zero in the struggle over a national health care overhaul pending in Congress, and groups on both sides of the issue have emerged to blast mostly negative ads at the relatively small electorate that will decide the election Tuesday. Total spending on advertising in the final nine days by the major-party candidates, Democrat Martha Coakley and Republican Scott Brown, and their allies could approach $3 million, which is an astounding figure for a Bay State election.

The saturation ad buys are most apparent in time slots adjacent to local news programs or in morning programs on the networks. For instance, viewers of ABC’s “Good Morning America’’ in the Boston market would have seen at least a dozen Senate campaign spots yesterday, most of them entirely negative, during the two-hour program on the local affiliate, WCVB-TV.

Today, at least 11 spots are scheduled, and tomorrow there will be at least 13, a review of the station’s public files of political ads reveals.

Spots attacking Brown often run back to back, as the Democratic state and national parties and outside groups backing Coakley, such as the Service Employees International Union (SEIU), attempt to halt the Republican’s surge in the polls and in fund-raising, in a race for the seat held by Edward M. Kennedy for nearly 47 years, once thought to be impregnable to Republican challengers.

Brown’s declaration that he would be the Republicans’ 41st vote to kill the health care bill in the Senate nationalized the race almost overnight.

“It’s a goal-line stand for the Democrats,’’ said Ray La Raja, professor of political science at the University of Massachusetts Amherst. “The stakes are so high now, with this being the 60th seat the Democrats need. . . . They’re scared out of their minds. President Obama doesn’t want this to be a referendum on his health care policy, especially in Massachusetts, which has the archetypal policies he’s trying to use in Washington.’’

What was supposed to be a cakewalk is suddenly a strain on the party treasury, draining funds that otherwise would have been available to defend vulnerable Democratic seats this fall.

At the national and state level, the Democratic Party will expend the maximum $880,000 allowed in coordination with the campaign of Coakley, state party chairman John Walsh said yesterday.