The credibility of next September's America's Cup defense took a major step forward yesterday when Dennis Conner announced he is reactivating his Stars & Stripes team, which won the Cup last year, and will personally skipper the defender.

Conner's role in battling a controversial challenge by New Zealand banker Michael Fay was cloudy until now. He's been "very ambivalent," said John Marshall, design coordinator for the Conner campaign, "but it was obvious the team needed a man to rally around who had the experience to carry us through this."

Some San Diego insiders even had suggested Conner might take no part in the sailing, but at a news conference in New York the veteran of five Cup efforts had harsh words for the challenge, which he called "an ambush," and warned, "Americans do not respond passively to sneak attacks."

Fay won the right to a head-to-head showdown for yachting's crown jewel in huge, 90-foot-waterline boats when he challenged Conner's San Diego Yacht Club last July under literal terms of the century-old Cup Deed of Gift.

Conner's forces, planning a multinational Cup defense in 1991 in traditional 12-meter yachts, initially ignored the offbeat challenge, but were ordered in November to meet it when Fay appealed to the New York State Supreme Court, which administers the Deed.

Racing now is scheduled to start Sept. 15. San Diego Yacht Club has not named a site, but sources say it may well be windy Hawaii, where Conner trained for his 1987 Cup triumph.

Conner's fiery rhetoric in New York, along with an announcement Pepsi-Cola had donated a reported $2 million, the first corporate sponsorship for the financially struggling defense, breathed life into the U.S. effort, which had been mired in the court proceedings and recriminations against Fay.

But Marshall said Conner's team will be playing a treacherous game of catch-up. The New Zealanders already are building their challenger and expect to launch in March. At twice the length of a 12-meter, it will represent a new and untested concept in yacht racing that will take time to master -- an immense, ultralight single-masted vessel bigger than anything raced in half a century and capable of speeds three or four times those of a 12-meter.

Marshall said his design team will test even more radical concepts, including hydrofoils and multihulls, and will labor under intense time constraints to get a boat in the water by July 1 and have it up to racing snuff two months later.