So many independent interest groups are poised to spend large sums on advertising to influence elections this year that Republicans and Democrats alike fear the candidates may find themselves playing bit parts in their own campaigns.

The first congressional campaign of 1998, here on the central California coast, offers a preview. Flip on the television or radio and it sounds in the advertising as if the contenders in Tuesday's special election want to talk about term limits or abortion.But the candidates themselves insisted in interviews that they would prefer to debate other issues, like education and taxes. Instead, many commercials and direct-mail drives are being orchestrated by outside groups whose officials know nothing about Santa Barbara except what they might have seen in the defunct television soap opera bearing its name.

"Is this the future of campaigns?" asked Tom Bordonaro Jr., a Republican state assemblyman who is seeking the seat. "Pretty soon, all we're going to have to do is file and sit back and let all these independent expenditures run the show. It's going to be rolling the dice whether you get into office or not."

By paying for their own commercials rather than donating directly to a candidate, these groups find that they have more control over the campaign discourse. They can also skirt federal contribution limits and disclosure regulations by trying to help (or, more often, hurt) a candidate with their own advertisements.

Spending by these groups rivals candidates' own television budgets. Their commercials frequently name candidates directly and are often difficult to distinguish from those written, produced and paid for by the campaigns.

Interest groups were instrumental in helping Republicans win back Congress in 1994, and they proliferated in the 1996 campaign. But political strategists, campaign finance experts and officials of the groups themselves said they expect this to be a pivotal year in which groups from both left and right transform politics by dominating races at all levels around the country.

Gary Bauer, a prominent conservative who last year set up an anti-abortion organization, the Cam-paign for Working Families, said the group was pumping $100,000 into television commercials in the district this weekend to help Bordonaro. Bauer said the special election here was only a precursor.

"We intend in the months ahead to be in races all over," Bauer said, adding that his efforts here have given the late-term-abortion issue "a much higher profile than it would have had" in the race.

By dictating the agenda on the issues, these outside groups could bring about a nationalization of elections that have traditionally been fought at the local level. Rather than argue over matters of intense concern to local voters, these groups encourage a bitter partisan debate on issues that may be of greater consequence in Washington.

Political parties have also been spurred to mount their own independent campaigns in the wake of a U.S. Supreme Court ruling in 1996 that the federal government may not limit how much the parties spend to help candidates - unless the party and candidates are working together.

"Our sense is that both parties will engage in running issue ads at all different levels of the federal elections," said Dan Sallick, communications director of the Democratic Congressional Campaign Committee, which helps elect Democrats to the House. "In 1998 people are much more familiar with the law and will make an effort to run ads that benefit the party through talking about the issues we care about most."

Yet Sallick said the campaign committee was so concerned about the spending by these groups that it was urging candidates to devote even more time raising money so they can compete. "We counsel a lot of candidates to be more aggressive in their fund-raising efforts," he said. "We want them to have the opportunity to get their message out."