Leading Senate Democrats warned yesterday that President Reagan's executive order on federal regulation could slow the regulatory process dramatically and could insulate administration decision-making from public and congressional oversight.

The senators said that provisions of the order -- which gives the Office of Management and Budget additional power to intervene in federal regulatory proceedings -- "may undercut the ability of Congress and the public to hold regulatory decision-makers accountable." They also said that the OMB oversight process will be conducted in secret and, as a result, "certain segments of the public could bring a strong influence to bear during the OMB clearance process, while others will be frozen out."

The analysis was released in conjunction with testimony before a House Judiciary subcommittee by Sen. Thomas Eagleton (D-Mo.), chairman of the Senate Democratic task force on regulatory reform. The hearing was the opening session in this year's congressional debate on regulatory reform legislation.

In prepared testimony, Eagleton sounded a warning against cost-benefit analysis, the controversial tool that is a centerpiece of administration efforts to curb federal regulation, saying that strict application could scuttle emergency regulations.

"At a minimum, these approaches could add needless delay and expense to the administrative process," Eagleton said. "Worse still, necessary regulations in such areas as nuclear safety and toxic-waste protection could remain unissued merely because the value of avoiding atomic catastrophe or future genetic damage could not be precisely toted up in an accountant's ledger."

Eagleton said the task force -- which is made up of seven Senate democrats, including Sens. Edward Kennedy (Mass.), Carl Levin (Mich.) and Lawton Chiles (Fla.) -- also is concerned about the legislative veto and general congressional oversight issues.

Eagleton said he opposes the veto concept, which would permit one house of Congress to overturn agency rules, but said the task force is "edging" toward support of a "1 1/2-house" veto, like that proposed by Sens. Levin and David Boren (D-Okla.)

The legislative-veto debate is likely to be a major congressional battle during the current session. More than 160 House members already have endorsed legislation by Rep. Elliott Levitas (D-Ga.), and a similar bill introduced by Sen. Harrison Schmitt (R-N.M.) also has broad support.

But the Reagan administration said in a statement released last week by Attorney General William French Smith that it opposes legislative vetos when the power intrudes on presidential power to "manage" the executive branch.

Smith said that the administration would consider legislative-veto bills on a case-by-case basis, but that as a rule the administration considers such proposals unconstitutional.

The statement irked some members of Congress, particularly in light of a Reagan campaign pledge to support legislative-veto plans.