Americans drink more soft drinks than tap water, some experts say, and the industry that serves up those soft drinks is on the verge of a potentially monumental change.

Coca-Cola Co., the industry leader, wants to buy Dr Pepper Co., the fourth-largest soft-drink maker. And Coke's archrival, PepsiCo Inc., is set to buy third-ranked Seven-Up Co. Within a few weeks, the Federal Trade Commission is expected to pass judgment on these proposals, which add up to the greatest challenge ever to the historic argument for antitrust regulation by the government.

Coca-Cola's $470 million offer for Dr Pepper would give the combined companies 46 percent of the soft-drink market, according to Jesse Meyers, publisher of Beverage Digest.

Another 36 percent of the market would belong to Pepsi if its $380 million purchase of Seven-Up goes through, Meyers estimates.

Together, then, these two giants would control 80 percent of the soft-drink-manufacturing business.

Not very long ago, mergers creating market shares on that order would not have been considered, much less proposed, because of the certain opposition by the FTC or the Justice Department's antitrust division.

The historic presumption by regulators was that, when a few very large companies dominate a market, they are apt to fix prices above competitive levels or drift into other anticonsumer practices even if they don't connive directly. Section 7 of the Clayton Act was a potent safeguard against that danger, authorizing the government to challenge mergers that might tend to lessen competition.

Some critics of the Coke and Pepsi deals say that is just their concern in this case. The mergers would "promote price-fixing and other anticompetitive behavior by the [soft-drink] manufacturers . . . ," contends Sen. Howard H. Metzenbaum (D-Ohio), one of the Senate Judiciary Committee's few surviving hawks on antitrust policy.

"Even under the most liberal antitrust guildelines issued under this and previous administrations, these proposed mergers are clearly unlawful," said James W. Harralson, chief operating officer of Royal Crown Cola Co. Royal Crown, with only 5 percent of soft-drink sales, would be the industry's largest remaining independent if the mergers go through.

The soft-drink deals raise the question of whether Section 7 "is alive or dead," said David Foster, an attorney with Willkie, Farr & Gallagher, who represents Royal Crown Cola.

"If the FTC doesn't go after these deals, there is no illegal deal under Section 7," said Foster.

But the economists and economics-minded lawyers who hold the key regulatory positions in the Reagan administration argue that it's time to take a more realistic look at competition and the Clayton Act's so-called safeguards.

Mergers of giant firms are not obviously bad if they lead the combined companies to improve efficiency and output, increase product choices for consumers and stir up competition, Daniel Oliver, newly appointed chairman of the FTC, said in testimony before the Senate Judiciary Committee last month.

The "old thinking" about competition and the Supreme Court cases that echoed that theory in the 1950s and 1960s were flawed, because they assumed that the best way to protect consumers was to keep lots of competing firms in business, Oliver said.

Under the Reagan administration's version of the "new thinking," proposed to Congress as antitrust reform legislation, the key issue is whether a merger produces companies with the power to boost prices above competitive levels and keep them there by trampling smaller rivals or new competitors in the market. Those mergers should be stopped.

The test isn't essentially a numerical measure of how much of a market the largest firms control. Other questions have to be considered: How do the big firms behave? Are they cosy or do they battle each other? Are imports an important source of competition? Is the product in question vulnerable to competition from substitute products?

To conservative economists whose faith is in markets, not regulators, even these questions aren't likely to produce the right answers. What market should the FTC be looking at? asks Fred L. Smith, president of the Competitive Enterprise Institute. Is it colas, or bottled soft drinks or beverages of all kinds? The larger the market, the less the combined shares of Coke and Pepsi and the easier to justify the merger.

The efforts by the FTC staff to unravel these questions reportedly have produced a sea of documents from Coca-Cola, Pepsi and the other bottlers. FTC: Watch, a newsletter that keeps a close eye on the commission, said the filings may have broken the multimillion-page record of documents set a decade ago in the FTC's investigation of the oil industry.

The magnitude of the review has convinced some outside experts that the FTC staff is taking a hard look at both proposed deals. But the vote by the commissioners is still hard to predict.

Larry Jabbonsky, editor of Beverage World, an industry newsletter, sees evidence that, even without the proposed mergers, life is getting harder and harder for the industry's smaller fry -- both the few remaining soft-drink manufacturers and the independent bottlers. But, he adds, "I think the climate is such that the deals are going to happen."

Meyers, publisher of Beverage Digest, says he has "no idea" what the FTC commissioners will do. "My instinct is, it's not just an exercise" by the FTC staff, said Meyers. The commission's verdict may fall somewhere between outright approval and a lawsuit to block the mergers, he guesses -- "not a comedy or a tragedy." The mergers, if approved, may be restricted by detailed conditions the soft-drink producers would have to satisfy.

If the lawyers, economists and other experts knew more about the way competition works, the decision would be easier to write and the theory would be simpler. But neither the "old thinking" nor the "new learning" about antitrust policy has produced a fail-safe way to judge the benefits or harm from mega-mergers before they take place. If Meyers is right, the FTC's decision may well be a messy but unavoidable recognition of that quandry.