Two of the government's chief economic regulators faced off yesterday against an array of congressional Democrats intent on showing that the Reagan administration isn't meeting its responsibility to protect consumers and investors from price-fixing and stock market manipulation.

Appearing at separate House subcommittee hearings, Securities and Exchange Commission Chairman John S. R. Shad and Assistant Attorney General Douglas H. Ginsburg, head of the antitrust division, gave no ground to their critics.

Ginsburg went before the House Judiciary subcommittee on monopolies to defend the administration's far-reaching package of antitrust amendments. After a few words of welcome, Judiciary Committee Chairman Rep. Peter W. Rodino Jr. (D-N.J.) swung into an expected attack on the administration proposal, underscoring its doubtful prospects before his committee.

Rodino charged merger regulation "appears to be at its lowest level at any time since the 1930s." During fiscal 1985, out of 1,604 premerger filings by companies seeking to merge, the Federal Trade Commission and the Justice Department took enforcement action in only 11 cases, Rodino said.

Ginsburg replied that, although the number of mergers worth $1 billion or more has increased markedly, there has been no increase in the percentage of total assets or employment accounted for by the 25, 100 and 200 largest firms in the nation. If anything, the concentration of economic power is declining, Ginsburg said.

The Reagan administration isn't ducking a responsibility to challenge mergers by competing firms that threaten to facilitate price-fixing or other collusion in an industry, or to create a firm with significant market power, Ginsburg said.

But it does not assume that mergers -- even the billion-dollar mergers of the 1980s -- are intrinsically bad, he said. "In enforcing the merger laws, the government does not, and cannot, second-guess business decisions unless there is a demonstrable harm or threat of harm to competition."

Ginsburg was followed by a panel of critics, including a predecessor as head of the antitrust division, John H. Shenefield. He charged that the administration's antitrust reform proposals assume that the marketplace will cure most monopolistic practices, because a monopolist's high prices will permit new competitors to enter the market.

By cutting back on the antitrust division staff, reducing the penalties available in private antitrust suits and taking a more lenient stand on merger enforcement, the administration is placing a misguided, misconceived faith in the marketplace, Shenefield said.

One corridor away in the Rayburn House Office Building, Shad was defending the SEC's record of surveillance and enforcement before the House Energy and Commerce subcommittee on telecommunications, consumer protection and finance.

Shad's challenge came not as much from the subcommittee's Democrats as from a panel of critics called in by subcommittee Chairman Rep. Timothy E. Wirth (D-Colo.).

Richard M. Phillips, a Washington attorney, former SEC staff member and chairman of an American Bar Association committee on securities regulation, contended that the SEC staff is falling far behind in its enforcement and surveillance activities.

The pressures on the federal budget prevent a significant increase in SEC personnel, Phillips noted. While commending Shad and the SEC staff, Phillips said, "Productivity increases squeezed from existing staff, even if valid, cannot cope with 75 to 400 percent increases in potential workload in most facets of SEC regulation during this past decade." And given the growth of the market, "The worst is not over -- it is just beginning," Phillips said.

Subcommittee Republicans took after Phillips and other witnesses, challenging their conclusion that the tremendous growth in securities trading implied a comparable growth in potential fraud and manipulation.

Shad said there is no evidence of rampant securities fraud and declared that the U.S. securities markets are the strongest and fairest in the world. He warned that the long climb in stock market prices may have masked some abuse of investors in the sale of stock. "But the first line of defense has to be a leery investing public." The market is not riskless, he said.