The Justice Department has 30 days, which can be extended to 60 days, to review the merger and decide whether to challenge it in court under the Hart-Scott-Rodino Act, which Flynn helped draft in 1968 as an attorney to the U.S. Senate Judiciary Committee's Antitrust Subcommittee.

States and private parties also can challenge a merger.

The antitrust standards that mergers can violate are outlined in the Clayton Act, which "provides you only have to show that it will lessen competition or tend toward a monopoly," Flynn said.

If the government stalls in making a decision or if stock prices begin fluctuating while the government is making a decision, "people will get nervous and call it off," Flynn said.

Some mergers are successes. Some are failures. But Flynn warns that two merged companies never live up to the hype that accompanies news of their merger.

"Basically, they make all kinds of grandiose promises, a lot of which never turn out to be true," he said. "They're trying to get stockholders to buy into it and government to buy into it."

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E-mail: lhancock@desnews.com