A bipartisan political study group is asking Congress and the Federal Election Commission to close a loophole in federal law that enabled the Democratic and Republican national committees to spend at least $15 million on their general presidential election campaigns last year in addition to public financing.

The study was one of several moves this week relating to campaign financing. The FEC and the House subcommittee on elections are scheduled to meet Thursday on proposed changes in the finance laws. The Senate Rules and Administration Committee held hearings yesterday on a proposal to allow public financing of Senate general elections in an effort to curb the influence of special interests.

On the loophole issue, the Center for Responsive Politics (CRP) charged yesterday that the two major parties used so-called "soft money" to generally help their presidential candidates, although Congress intended it solely for party building, not candidate benefit. Under FEC rules, such soft money is not regulated in terms of limits on individual contributions and disclosure.

The CRP estimated that the Democratic National Committee funneled about $10 million in "soft money" into Walter F. Mondale's campaign and that the Republican National Committee put about $5 million into President Reagan's reelection effort.

Congress enacted the soft-money provision, which allows unlimited contributions to state and local parties, in 1979. The CRP and other critics charge that funds channeled from the national committees to state and local organizations often are used to free up local money to support federal candidates.

The federal campaign laws put a $20,000 annual limit on individual contributions to the national committees. There is no limit on contributions that individuals, corporations and labor unions can make to off-budget or "soft money" accounts. Some contributors, such as former IBM chairman Thomas Watson and Minnesota department store heir Mark Dayton, reported giving $100,000 last year.

In the Senate hearings, several lawmakers called for public financing of Senate campaigns in order to eliminate the influence special interests can have on candidates through their political action committee contributions. Sen. Gary Hart (D-Colo.), saying the public perceives such contributions as thinly disguised bribery, called for cleaning up "the toxic waste of American politics."