The National Right to Work Committee, an organization opposed to union shops, is spending $100,000 this year to secretly place an estimated 100 private investigators in union political operations, voter registration organizations and other groups.

The investigators, many of them employed by a Falls Church detective agency, Miller and Associates, have infiltrated the groups by pretending to be supporters willing to volunteer their services, by obtaining paid jobs and by portraying themselves as free-lance reporters.

"I certainly see the need for it," Howard Miller, a former District of Columbia policeman who heads Miller and Associates, said. "If we don't do it, all political organizations are going to run amok."

Reed Larson, president of the committee, said the private investigators have been placed in many unions affiliated with the AFL-CIO, the National Education Association and in such groups as Project Vote, an organization specializing in the registration of poor people.

Larson said he sees nothing unethical about the tactic of secretly placing private detectives in political, union and civic organizations.

Sanford Newman, president of Project Vote, disagreed. "It is conduct which is clearly outrageous, clearly unethical, and quite probably criminal," he said. "It is an outrageous invasion of privacy."

Miller said: "We are a regulated industry and I would rather have a regulated industry doing this than having the Watergate boys getting together for fun and games."

Miller said his organization is regulated by the state of Virginia and the District of Columbia.

The purpose of the secret infiltration is to identify alleged violations of federal election law and misuse of funds by unions, the Mondale-Ferraro campaign and by tax-exempt organizations, according to both Larson and Miller. The central focus has been the Mondale campaign and organized labor.

Larson said the $100,000 expenditure for the private detectives is to root out alleged criminal activity, and amounts to a political expenditure. He said the Right to Work Committee has an annual budget of about $4.5 million.

Most recently, the Right to Work Committee used three Miller and Associates investigators to infiltrate Project Vote in an attempt to build a case that it is "masquerading as a nonpartisan, good-government endeavor while, in fact, it is serving as a partisan front for the Mondale campaign," Larson said.

On the basis of its findings, the Right to Work Committee filed a complaint with the Federal Election Commission and wrote to the Internal Revenue Service charging that Project Vote should lose its tax-exempt status.

The committee's charges are primarily based on affidavits from the Miller and Associates detectives in which Project Vote workers are quoted making statements sympathetic to the Democratic Party and to Democratic candidates.

The most serious charge against Project Vote is made by an investigator named Rhoda E. Brown, who claimed she was instructed to delete newly registered voters "who had said they were Republicans" from a Pittsburgh list apparently being prepared for get-out-the-vote drives.

Newman contended that there is no way that Republicans could be selectively eliminated from the lists of voters registered by Project Vote. He said that the various pro-Democratic quotations attributed to members of the Project Vote staff amount to an allegation "that some of our staff have political views," which Newman did not dispute.

Newman said one of the investigators, Robert H. Hemmig, came to Project Vote describing himself as a free-lance reporter. Hemmig could not be reached for comment. Miller said Hemmig is both a licensed investigator and a "credentialed reporter."

Miller said all his operatives comply with the law, which, he said, prohibits private detectives only from pretending to be government officials and police officers.