Sen. Robert W. Kasten Jr. (R-Wis.) failed to file state or federal income tax returns for 1977, when he was a congressman.

When Kasten discovered last summer, while he was running for the Senate, that he hadn't filed a federal return for 1977, he quietly filed one. He didn't, however, tell the public about it. In a clean-government state like Wisconsin, people take things like failing to file income tax returns seriously, and such an admission certainly would have damaged Kasten's election chances against former senator Gaylord Nelson, a liberal Democrat.

The failure to file might never have become public except that Wisconsin has a law allowing people to ask the state Department of Revenue how much anyone has paid in taxes. A reporter for the Madison Capitol Times made a routine check on a number of public officials, and found Kasten hadn't filed a 1977 return.

Under questioning, the Republican senator acknowledged he hadn't filed his federal return for that year until 1980, more than two years late. He said tax had been withheld from his congressional salary during 1977, however, and he received a refund of more than $4,000.

Kasten initially was certain he had filed the state return, and his office issued a press statement describing it all as "an unfortunate misunderstanding."

But after searching his records, a red-faced Kasten announced he hadn't paid his state taxes, "though an administrative mistake by me, my secretary/bookkeeper or my accountant."

"I am embarrassed by this mixup and it is my understanding that the revenue department will bill me for any interest or penalty due on the portion of my 1977 tax liability that was in question," he said in a statement issued to the Wisconsin press.