Washington ? Political parties, candidates and interest groups spent about $1 billion on television advertising for the midterm congressional elections, racing to use up reserves of now-banned campaign contributions.

A campaign finance law that took effect Nov. 6, the day after the vote, prohibits national party committees from raising and spending soft money — the unlimited contributions from corporations, unions and others that parties use for activities such as issue ads, get-out-the-vote drives and operating costs.

“There were so few races in play that there was a lot of money chasing a few races,” said Ken Goldstein, a political scientist who is director of the Wisconsin Advertising Project. “Advertising matters at the margins, and can influence 2, 3 or 4 percentage points of the vote.

“But we are living in a time where politics and elections are decided at the margins.”

The United Seniors Assn., a group backed heavily by pharmaceutical companies, spent $8.7 million for television ads, with much of that supporting Republican House candidates. That was almost three times more than any other group spent on campaign advertising.

United Seniors spent the money on more than 15,000 ad spots in 35 markets.