WASHINGTON — Would American taxpayers donate $3 to ensure that their U.S. senators are not crooks?

Sen. George J. Mitchell thinks the answer is yes.

Flanked by a half-dozen prominent Democrats, Mitchell detailed a sweeping reform bill Thursday that would impose limits on the amount of money that members of the Senate may spend on their campaigns, prohibit the use of special-interest Political Action Committee (PAC) contributions, and attempt to persuade radio and television stations to offer candidates discount air time to broadcast longer, more substantive commercials.

The Democratic plan would be funded by a taxpayer checkoff system similar to the one used in presidential elections.

Republican Senate leaders outlined a competing campaign reform bill last week that also banned PAC contributions, but skirted the issue of a campaign spending cap.

The haste with which both parties seem to be moving toward campaign finance, some lawmakers admitted, has been accelerated by political scandals involving some of the Senate’s most well-known members.

Sen. Joseph Biden, D-Del., made no bones about the linkage, telling reporters that he would rather have a challenger face him with equal financial resources paid for by public funds than have his personal reputation destroyed by the sort of allegations being levied at senators currently involved in the savings and loan and Housing and Urban Development scandals.

Sen. William S. Cohen is viewed as one of the swing members of the Senate on the campaign finance issue. He is one of 11 Republican senators — there are 45 — who has not endorsed his own party’s bill and has been talking with key Democratic leaders about a compromise proposal that would meld the best aspects of both party’s legislative proposals.

Under the Democratic plan, Senate candidates in Maine could spend no more than $1,586,500 for primary and general elections. The cap varies from state to state, based on population.

During past debates, Cohen has opposed a ceiling on campaign spending, saying such a provision would place challengers at a disadvantage with incumbents. Kathy Gest, Cohen’s press secretary, said that Cohen no longer is adamantly opposed to a spending cap.

For Mitchell, a spending ceiling is the key stumbling block on the campaign finance issue.

“The only meaningful way to reform the Senate election finance system is to have limits on campaign spending. Anything less than that avoids the real issues and simply creates the appearance of reform,” he told reporters.

According to Mitchell, the cost of Senate races averaged $4 million in 1988, forcing incumbent senators to raise “$13,000 a week, 52 weeks a year, for six years.”

“The system by which American campaigns are financed is out of control, and getting worse. It does not serve our nation’s best interests,” Mitchell said.

Under the Democratic plan, Senate candidates could, if they met certain qualifications, have up to 70 percent of their general election costs paid by the taxpayers.

That money would come from voluntary $3 contributions made by taxpayers on their annual tax forms. Senate aides said that unlike the presidential campaign financing system, the Democrats’ bill would require that if a person agreed to give the money, it would be added to that person’s tax bill.

Under the presidential system, a taxpayer can designate $1 for the fund, but that does not raise or lower a person’s taxes.