The Reagan years have witnessed one of the greatest waves of mergers, takeovers and corporate restructurings in history, with the tally standing at more than 25,000 deals worth $2 trillion since 1980 and still rising.

Last year, 25 of those mergers were challenged by the two agencies responsible for policing the marketplace, the Federal Trade Commission and the Antitrust Division of the Justice Department. Many of the deals were the type of transaction that would not have been blocked on antitrust grounds by any administration.

But overall, antitrust lawyers said, the door to corporate mergers has never been open wider than during the Reagan presidency. And most experts expect it will close at least partially no matter who wins the Nov. 8 presidential election.

"Most antitrust attorneys believe that there has been an opportunity {under the Reagan administration} to do deals that wasn't there before," said a Washington attorney who represents a grocery chain involved in a major merger bid.

To the president's supporters, the change in antitrust policy is a triumph of the administration's battle to control government regulation and is a victory for conservative faith in free markets.

"The intellectual view of antitrust has changed immensely, not only in the enforcement agencies, but also on the Supreme Court," said Robert H. Bork, a former U.S. Court of Appeals judge who is now an American Enterprise Institute scholar and one of the architects of the new policy.

But to the administration's sharpest critics, the new policy is an abdication of regulatory responsibility. The deals that have gone through, they said, may result in less competition, fewer product choices for consumers and unjustified price increases.

"Many of these massive combinations -- in oil, steel, airlines and other basic industries -- would never have passed muster under any other administration, be it Democratic or Republican," said New York state Attorney General Robert Abrams, who joined this year with colleagues from the other 49 states in criticizing the administration's policies.

The FTC is now "a gaunt and bloodied agency" that is rapidly losing its remaining strength because of reductions in its budget imposed by Congress and by the administration, said FTC Commissioner Andrew J. Strenio Jr., a Democrat.

"Since fiscal year 1980, there has been a drop of more than 40 percent in the work years allocated to antitrust enforcement. In the same period, merger filings skyrocketed to more than 320 percent of their fiscal 1980 level," Strenio said in August.

In the administration's defense, Assistant Attorney General Charles F. Rule, head of the Antitrust Division, said the government's weapons have been concentrated on the most important targets -- illegal price fixing, bid-rigging and other criminal conspiracies.

"The simple truth is that it has never ... been riskier and more dangerous to commit an antitrust crime," Rule said in a speech Thursday.

"To hear our critics, one would have to conclude that we are asleep at the switch -- that we have failed to stop all sorts of anticompetitive mergers," Rule said.

But the critics cannot point to one merger the government failed to block that has led to higher prices for consumers, he said.

Whether the public is content to let the marketplace govern regulatory policies will be tested again soon, when a new administration takes office. A healthy, growing economy would help validate the Reagan policy.

But if the economy falters during a George Bush or Michael Dukakis administration, critics will find it easier to say the Reagan deregulatory pendulum had swung too far.

"There is a danger," said Bork, "that the gains {of the Reagan antitrust policy}, which are real gains for consumers, will be taken away by populist legislation, particularly if there is a Democratic administration, or a Republican administration that doesn't care."

"The administration has moved radically to the right," said Robert Pitofsky, dean of Georgetown University's law school and a former FTC commissioner in the Carter administration.

"The pendulum will swing back somewhat regardless of whether George Bush or Michael Dukakis is elected," he said.

As the American economy has grown more complex and interconnected with foreign markets in the 1980s, the problem of assessing the impact of mergers has become far tougher than it once was.

Since the great merger wave near the turn of the century, which produced the first major antitrust law -- the 1890 Sherman Act -- the goal of antitrust laws has been to prevent giant companies from dominating smaller rivals and to head off attempts by competitors to fix prices, carve up markets or commit other anticompetitive acts.

The antitrust laws, however, speak in generalities, more like New Testament parables than the Ten Commandments, and have left plenty of room for enforcement of the laws to swing with the political currents.

When liberal or populist views dominated, regulators and judges tried to break up relatively small mergers, fearing no good could come of them. Thus the Supreme Court in 1962 blocked a proposed merger of the nation's third- and eighth-largest shoe retailers, even though together they would have controlled less than 10 percent of the market.

Four years later, the court, citing its concern over a trend of mergers in the grocery business, barred a merger of the third- and sixth-largest grocery chains in Los Angeles, which together accounted for 7.5 percent of grocery sales in the area. The best way to ensure strong competition is to have a lot of competitors, the court concluded.

The older, liberal-populist view of antitrust was based on a fear that once a few companies gained control of a market, they could collectively squeeze smaller rivals into the corner and share unfairly high profits.

In the 1970s, Democrats in Congress sought to enact tough formulas for viewing competition. When a small number of firms controlled 50 percent of a market, the firms would have to be broken up into smaller units, unless they could prove they had to be big to be efficient, some Democrats proposed.

But conservatives, moving into power in the early 1980s, responded that such formulas didn't make sense in an economy awash in imports, in a world where technological change can make entire product lines obsolete in a few years, and in a marketplace where foreign investment, takeovers and buyouts are constantly changing the players.

Washington attorney Ernest Gelhorn said the Reagan revolution in antitrust policy was really part of a continuing adaptation of the law to the changing realities of the marketplace.

Likewise Bork saw the evolution as a move away from heavy-handed, "government-knows-best" interference in the marketplace that was rooted in ignorance of how the economy really works. Being big, by itself, in the corporate world is not automatically bad, in this view. "There is nothing written in the sky that says the world would not be a perfectly satisfactory place if there were only 100 companies," said William Baxter, Reagan's first head of the Antitrust Division.

Nor are mergers automatically evil, said Rule. Most "are driven by objectives other than raising price," for example, ridding a company of inept or inattentive management, improving efficiency, meeting changes in market demand, responding to foreign competition or taking advantage of tax laws.

The true purpose of antitrust law should be to provide the best deal possible for consumers, according to Bork and the conservatives. Viewed in that light, mergers and agreements among competitors, or between manufacturers and retailers, might benefit consumers even though they also benefit big, powerful companies if they make the companies more efficient. And in that case, they shouldn't be opposed, the conservatives said.

The new policy means size and market power are no longer red flags that automatically bring companies under suspicion. One good example of the new policy at work is a May Supreme Court decision. In the past, any discussion between a manufacturer and a retailer about setting store prices for the manufacturer's goods was illegal. Then came a complaint challenging the way Sharp Electronics Co. allowed calculators it manufactured to be priced.

Sharp's longtime retailer in Houston consistently sold the calculators at discounts below the manufacturer's suggested minimum retail prices. When a second retailer protested to Sharp about this cost-cutting, Sharp agreed and cut off shipments to the discounter. The court allowed that action to stand.

As Bork explains the decision, it meant Sharp should be able to choose whether it wants its products sold by a no-frills discounter or a full-service retailer charging higher prices but offering trained salespeople to help customers understand the product. If the manufacturer makes a wrong decision, the customers will go elsewhere: Let the marketplace decide.

"I think Reagan struck a nerve," said Pitofsky, the Georgetown dean. "There is considerable validity to his claim that business was overregulated in many areas. People came to agree that there was too much 'snail darterism' -- too much regulation without any sense of the consequences. One consequence of the Reagan revolution is that in the future, we will regulate much more cautiously, with a much greater sense of what the costs might be.

"My quarrel with the Reagan revolution is that, like most revolutions, it went too far. Merger policy is a good example. The conservative critics of antitrust policy are absolutely right when they say we were wasting resources and costing the country money by challenging mergers at a microscopic level.

"But I see no reason why we should have policy that regularly lets through mergers among companies with 20 percent or more of a market, where there are high barriers to entry by possible new competitors. It stems from a faith that the market will take care of all problems eventually. There might be more than a bit of truth in that, but when does 'eventually' come about?" Pitofsky asked.