President Reagan's nominee to chair the Federal Trade Commission yesterday criticized as "misdirected" peices of the agency's antitrust record and said that upon confirmation he would make a "thorough evaluation" of the federal government's antitrust laws and their enforcement.

James C. Miller III, administrator of the Office of Management and Budget's Office of Information and Regulatory Affairs and director of a White House task force on regulation, told a Senate Commerce Committee confirmation hearing that he thinks the FTC should concentrate more on horizontal antitrust cases and not on "bigness-is-bad theories."

But Sen. John Danforth (R-Mo.) firmly told Miller that he has a responsibility to enforce antitrust laws unless Congress revises them, particularly the 67-year-old law that defined the commission's antitrust mission.

"The drift of this administration," Danforth charged, "is perhaps to turn the clock back, if not to Adam Smith, at least to 1914. . . . I am not encouraged by what you are telling me."

Danforth said the administration seemed to be moving the economy "in the direction of not only less regulation but less competition, as well. One of the hall marks of Republican doctrine is a competitive marketplace." Questioned by Danforth, Miller said he has no understanding with the White House about dismantling the FTC's Bureau of Competition.

The issue is particularly delicate because OMB Director David Stockman proposed earlier this year that the Bureau of Competition be phased out over three years, ending the FTC's original antitrust mission. William Baxter, assistant attorney general for antitrust, also has said he favors elimination of the FTC antitrust enforcement division.

But the administration dropped the plan in the face of small-business and congressional opposition to the dismantling.In fact, Sen. Robert Packwood (R-Ore.), the Commerce Committee chairman, personally told President Reagan that he opposed the move, in a meeting that led to the decision to reverse the plan.

Miller is slated to take over the agency's chairmanship in September after the term of FTC member Paul Rand Dixon expires. He was nominated to serve a seven-year term, and he is expected to win Senate confirmation easily.

Miller's nomination was sharply criticized by Mark Green, former director of Congress Watch, who said he was testifying on behalf of himself and Ralph Nader. "Mr. Miller's repeatedly demonstrated deference to the interests of business groups makes him strikingly unsuitable for the chairmanship," Green said.