Former Democratic Rep. Gunn McKay is confident he will have no trouble picking up where he left off if he returns to Congress after an eight-year hiatus.

But an aide to incumbent Republican Rep. Jim Hansen says that if McKay does go back to Washington, he will be ineffective and frustrated in an institution where attitudes and faces have changed in recent years.The question may be academic. McKay must first overcome an opponent who unseated him in 1980 and foiled his comeback attempt two years ago. Recent polls have given Hansen a 6 to 9 percentage point lead over McKay in the 1st Congressional District race.

McKay's advisers hope to turn that around during the final weeks of the campaign with heavy television and radio advertising. Even if they're successful, McKay will be in for a rude awakening, said Hansen campaign manager Peter Jenks.

"He would be a 64-year-old freshman congressman with not much of a future,," Jenks said. "Gunn McKay would get second-rate committees and feel not very much at home in Congress today."

McKay, who was elected to his first congressional term in 1970, doesn't buy that.

"We'll have more influence when we get there than he's had in eight years - the first day," McKay said.

Rep. Wayne Owens, the only Democrat in Utah's five-member congressional delegation, said he had no trouble adjusting when he returned to the House two years ago after a long absence. Owens served a term in Congress beginning in 1972 and was elected again in 1986.

Owens said McKay's reputation and popularity would make him a player from the start, and though he no longer would have seniority, he would have the advantage of being in the majority party.

"The House of Representatives is in fact a partisan body," Owens said. "Republicans have a terrible time getting anything done because the Democrats ignore them. I'm not advocating that that's good policy, but that's the way it is."