NASHVILLE, JUNE 16 -- Television evangelist and prospective presidential candidate Marion G. (Pat) Robertson today proposed replacing most federal urban aid programs with a system of voluntary contributions supported by a 10 percent income tax credit.

Robertson, a Republican, joined three declared candidates in seeking support at the 55th annual Conference of U.S. Mayors here. This was the only day of the five-day conference in which substantial Republican participation was planned.

Former Delaware governor Pierre S. (Pete) du Pont IV denounced his rivals in the contest for the GOP nomination for promising federal funds in a "bidding war" for big-city support. He also outlined proposals for mandatory teen-age drug testing in schools and replacing welfare with mandatory work programs.

In contrast, the two Democratic candidates appearing today -- Sens. Joseph R. Biden Jr. (Del.) and Albert Gore Jr. (Tenn.) -- promised substantial restorations of domestic spending programs.

Robertson's tax proposal, which he acknowledged needs "some refinement," would give tax credits to corporations and individuals that contribute to local government projects.

Affluent communities with large numbers of middle- and upper-income residents or profitable, tax-paying corporations, or both, would benefit most. Since the system would depend on voluntary contributions, large taxpayers, particularly major companies, would have significantly more influence over municipal expenditures.

"Perhaps to them that have, more would be given," Robertson said, but he argued that the proposal would increase citizen participation because taxpayers would have to write checks specifically supporting local programs to qualify for the credit.

He cited New York City's aborted Westway highway project as an example: "If Mayor Koch can get the people {of New York} to contribute $6 billion to a couple of miles of road in New York, let him do it."

Du Pont later dismissed the Robertson plan, contending that the tax system should not be altered until the economic consequences of the 1986 tax revision bill are known. His own welfare reform proposal, however, would expand the earned-income tax credit for the working poor.

Du Pont would require all healthy welfare recipients to work -- at subminimum wage government jobs, if necessary -- and would supplement their salaries through a tax credit to raise their income to the poverty level. "Welfare checks would stop," he said. Mothers would be allowed six weeks maternity leave, but there would be "no exception {from work} for mothers with children under 6 or 3 or 2."

Du Pont criticized the highly touted welfare reform program of a leading Democratic candidate, Massachusetts Gov. Michael S. Dukakis, who spoke here Monday.

"No wonder the welfare rolls of Massachusetts have grown," du Pont said, contending that the growth resulted in a failure to require work. The program's key feature is job training and placement for welfare recipients.

Biden and Gore called for new or restored federal spending on education, housing and health care. Gore proposed to initiate a "Manhattan-style" project to seek a cure for acquired immune deficiency syndrome and said the school year should be expanded from 170 days to 210 days. Biden called for free medical care for poor children under age 12, a doubling of Head Start funds and a new higher education program for poor college students.

The only major figure from the Reagan administration to speak here was Housing and Urban Development Secretary Samuel R. Pierce Jr. Fewer than 30 of the 200 mayors here listened to his defense of administration policies. "You as mayors have responded {to those policies} with ingenuity . . . realistically, if not enthusiastically," he said.