A Wisconsin legislator introduced a novel bill recently for public financing of his state's congressional elections, hoping to reduce the power of special interests and single-issue groups and to curb excessive spending in those contests.

Rep. Bill Broydrick said he expects the fight for enactment to be tough. But, the Milwaukee Democrat said, he is encouraged because his bill closely parallels a 1977 law affecting candidates for state, not federal, offices.

The law, which offers state candidates strong inducements to accept partial public funding, led to a 50 percent decline in spending by politial action committees (PACs) and to a 17 percent drop in candidate outlays.

The new Congress, which has many members who owe their election to contributions from increasingly numerous and heavily bankrolled PACs, is unlikely to pass legilsation to curb the influence of special interests in federal elections, said Broydrick, a former aide to Rep. Les Aspin (D-Wis.). He described private financing as "corrupting."

Spending in Wisconsin's nine House races in 1980 totaled $1.5 million, up 59 percent from 1978. Combined spending in the most costly race was $480,000.

Under Broydrick's bill, a congressional candidate would receive a probable maximum of $60,000 in public campaign funding. The maximum would be available to rival candidates who receive more than 6 percent each of a primary vote total.

Candidates must also accept the bill's ceilings on overall spending, agree not to take PAC money, raise specified minimum sums in individual donations of less than $100 each and limit contributions of their own money to their campaigns.

The 1977 law lets a candidate for the state Ssembly draw up to $5,310 in public funds. If he accepts PAC money, he must forfeit a dollar in public money for every dollar he takes from special interests. Rival candidates who accepted public funds in 1980 were under a spending lid of $11,800 each.

The law pays for public election financing with a voluntary checkoff of $1 for an individual and $2 for a married couple. That poured nearly $1 million into the Wisconsin Clean Campaign Fund in 1980. The checkoff, under Broydrick's proposal, would also pay for pubic financing of congressional races.

The law led to a drop in PAC contributions in state Assembly races from $534,027 in 1978 to $263,331 in 1980. Spending by the candidates fell from $1,110,249 to $916,373.

The Midge Miller of Madison, a member of the Democratic National Committee, said State legislators who opposed public financing admitted privately that they were troubled most because it helps challengers.

"The Congress is exactly the same way," she said. "The incumbents don't want to fund challengers.