The Senate bill overhauling campaign finance, approved just before the current recess, contains key provisions that may be unworkable and might be found unconstitutional, according to specialists on campaign finance.

They question some of the legislative language that spells out a complicated system under which federal funds would be provided to candidates who agree to spending limits. They also question the constitutionality of labeling requirements that would be imposed on candidates who refuse to accept those limits and on those who spend money in support of a candidate independent of the candidate's campaign.

The specialists also find objectionable a portion of the bill that calls for providing public funds to opponents of candidates who are supported by such "independent" donors.

In addition, the specialists, including some who helped draft the measure, said they question whether Congress will ever approve a way to collect and distribute funds to candidates who accept the voluntary limits, let alone administer and police the complex program. Candidates who accept the limit would raise 80 percent of their funds from contributors and receive the rest from the government under the Senate bill.

When Congress returns next month, Senate Majority Leader George J. Mitchell (D-Maine), the prime mover of the legislation, is expected to appoint conferees to meet with representatives of the House, which has passed a different campaign reform bill.

The Federal Election Commission thinks the Senate measure has so little chance of becoming law that it recently refused to provide the Office of Management and Budget with an estimate of what it might cost, according to an FEC spokeswoman.

In an interview, Jan Witold Baran, a lawyer here who served on a bipartisan campaign finance panel organized by the Senate leadership, cited several provisions that he said would raise constitutional issues.

The provision requiring that federal funds be given to the opponent of a candidate who is supported by an independent donor punishes that candidate even though the indepedent supporter, who may run an ad praising his candidate, is "exercising what the Supreme Court determined was a constitutional right," he said.

Baran said another provision that "has definite First Amendment implications" is one requiring "a full {television} screen personal appearance by the candidate . . . in which the candidate states: 'I am a candidate for {name of office} and I have approved the contents of this broadcast' " and paid for it.

Fred Wertheimer, president of the self-described public interest group Common Cause and a strong advocate of campaign reform, called the Senate measure "an excellent, but not perfect, bill."

Wertheimer noted that Common Cause "shares the view with others that there are serious constitutional problems" in the bill's attempt to ban political action committees, known as PACs. That was why a backup provision that only reduces the influence of PACs was made part of the measure, he said.

A top FEC official, who has been involved in past presidential and congressional campaigns, said the bill reads as if parts "were written by different staff members and none had been involved in campaigns."

He said it would be impossible for candidates to adhere to a provision that would require the FEC, after receiving a complaint about an independent expenditure in any Senate election, to launch a staff investigation outside Washington and make a commission finding in "not later than three days."

"It takes three days to get the FEC members together," said the official, who asked not to be identified.

He also criticized sections that would require the Treasury secretary on Jan. 1 of an election year to estimate how large a fund of federal money would be available for distribution to Senate campaigns that year and how much each candidate would be entitled to receive.

"At that point," the FEC official said, "you would not know" who was running, who would abide by the spending limits and how much independent expenditures would come into a campaign -- the key elements in determining how much money each candidate could get.

He also said independent donors normally appear in the closing days of campaigns. By the time their reports reach Washington, there may not be enough federal money left to send to opponents of an independent donor's candidate.

Wertheimer said he believes questions about how much federal money would be needed and how it would be disbursed can be answered. But he noted the Senate measure fails to establish a way to raise federal funding for the campaign financing program.

There was a plan to increase the size of the checkoff on income tax forms, which now permits taxpayers to direct $1 to a fund distributed in presidential elections, but because of opposition this proposal was not put to a vote, Wertheimer said.

Getting the $30 million to $50 million per election year that the bill's advocates say would be needed "has to be decided at a later point, separate from this legislation," Wertheimer said.

On the matter of television advertising, Baran said viewers may need additional notice of who is responsible for the campaign message, but the courts could require one "less restrictive" than the Senate provision.

A spokesman for Sen. John C. Danforth (R-Mo.), author of the "enhanced disclaimer" provision, said, "There is absolutely no First Amendment issue here that we can ascertain."

He said when a tougher rule was under consideration in 1985, it was reviewed by Newton Minnow, chairman of the Federal Communications Commission during the Kennedy administration, who reported "he was comfortable that it passes consititutional scrutiny." The Supreme Court, Minnow reported, recognized that the government has a role in enhancing the "ability of voters to make informed judgment through source disclosure."

Baran said another disclosure provision that would be "very difficult to justify because of its intrusiveness" involves television ads paid for by independent donors who act without authorization of a candidate's campaign. The bill would require that the entire advertisement carry a disclosure of who sponsored it in "a clearly readable video statement covering at least 25 percent of the viewing area of a television screen." The bill would require the same information to be announced at the end of the commercial.

Another provision in the bill that one Senate staff member said "is probably unconstitutional and would be dropped in conference" with the House would require each executive of a company who makes a contribution to a candidate to certify that the collective contributions of all executives of that company to that candidate did not exceed $5,000 or that their contributions to a party committee did not exceed $20,000.