Major U.S. Airlines Slice Summer Fares : Travel: Other carriers match or beat a discount coupon that United inaugurated.

The lure of a larger share of the vacation travel market has led the nation’s big airlines to break with tradition and discount some of their domestic ticket prices during the busy summer season.

United Airlines said Thursday it will meet the reductions of competitors who dropped prices in response to a $35 United coupon offered Monday.

The coupon is good on round-trip tickets of $175 or more.

Trans World Airlines cut some domestic round-trip fares Tuesday by up to 30%, and American Airlines, Delta Air Lines, USAir and Continental Airlines announced Wednesday that they also were lowering prices.

Industry analysts say that, so far, the fare cuts pose little danger to the companies’ earnings.

“There are some diminishing returns, as far as profitability, but you don’t give up market share to someone else,” said Paul H. Nisbet, an analyst with Prudential-Bache Securities Inc.

If the war escalates, earnings could become a casualty, he said.

Raymond Neidl, an analyst with Dillon, Read & Co., was surprised that airlines were offering discounts so early in the vacation season.

United’s coupons are good through Dec. 14, excluding Nov. 16-30. The offer requires a Saturday night stay, and tickets must be bought seven days in advance of departure and no later than Aug. 31.

United, which American surpassed last year as the nation’s biggest carrier in terms of traffic, was “looking for a recouping of that market share,” Nisbet said.

TWA responded by cutting domestic round-trip fares up to 30% for travel Tuesday through Thursday. TWA also is requiring a Saturday night stay and seven-day advance purchase. Tickets must be purchased by June 7. The discount is good for flights through Dec. 13.

Under TWA’s discounts, the lowest coast-to-coast fares fall to $298 from $428.50.

The restrictions place the lower fares out of peak periods for business travel, the carriers’ most lucrative market.