Dennis Conner's red yacht Liberty had to come from behind today but won her second straight America's Cup race as hapless Australia II, whose crew filed a protest, began to resemble the Australian Breakdown.

The Australians managed a remarkable feat as they stayed ahead of Liberty for half the race despite having their mainsail break free at the masthead just before the start. The big sail never drew properly all race and crewman Colin Beashel was hoisted to the top three times to try to strap it down and keep it aloft.

Crippled Australia nevertheless managed to stave off Conner's advances through 13 miles of the 24.3-mile course. But in the fourth leg of the six-leg race Conner ground down his opponents with a classic tacking duel as he crossed the wind 16 times. Australia followed suit the first 15 times, her injured mainsail flapping and the headboard swinging loose, but on the last tack she let the U.S. boat go off on its own.

Conner, a master at playing the wind, caught a beneficial shift and when the yachts came back together he quickly took the lead.

The Australians hoisted a red protest flag, claiming they were forced by Conner's aggressive actions at that point to tack away to avoid collision. An international jury will hear the complaint at 9 a.m. Friday, but in America's Cup racing protests are rarely upheld if there is no contact.

By race's end Conner had expanded his edge to 1 minute 33 seconds and held a 2-0 lead in the best-of-seven series for yachting's most coveted prize. But he still had not proved that Liberty is a faster boat than a fit Australia with her controversial winged keel.

"What I'd like to see," mused Australian Murray Davis, publisher of Cruising World magazine, "is a real yacht race where nothing breaks."

It was the second defeat by breakdown for Australia. Wednesday she battled head-to-head with Liberty for more than three-fourths of the race before a steering pulley gave way and she fell far behind.

Today's race was run under sparkling blue skies in a northeast breeze that started off stiff at 17 knots but had died to 10 to 12 by the finish. It was shifty, as well, moving east some 25 degrees by the end, and an angry Australian syndicate chief Alan Bond said that should have been cause for the race committee to abandon the contest.

Bond still was smarting because Tuesday's first race of the series was abandoned before its start because of shifty winds, with Australia firmly ahead. He maintained that had his crew not encountered the breakdown today, it would have won. And he expressed confidence the protest of Conner's tacking tactic would be upheld by the jury.

The Australians called a lay-day Friday, cancelling racing so they can regroup. But even that call was a source of controversy as Bond maintained he heard Liberty's crew call for the lay-day first, then had Australia raise its lay-day flag.

"We heard them call the lay-day (over the radio)," said Bond. "We called it four or five seconds later. It is our opinion they called it and if it is ruled the other way, we'll protest to the race committee." Each side gets one lay day during the first four racing days.

Racing resumes Saturday.

Despite this building sea of acrimony, a spectator fleet of about 800 boats got to see another stirring matchup on the water.

Conner was superbly patient as he rounded the course trailing the obviously troubled white yacht. "He sailed with a great deal of poise," said sail trimmer John Marshall. "I've seen Dennis at his best but out there he was better than his best."

As Conner sailed, said Marshall, "He constantly measured our progress against theirs. He was really sailing both boats, and when he saw that they were getting in trouble he put the pressure on."

That was on the fourth leg, the second time the yachts turned into the wind. And it was then that Conner, still trailing, began the grueling tacking duel, sending his boat through the wind again and again and forcing Australia to do the same to keep Liberty covered.

"We got Australia to try to cover us, tack for tack, and managed to get them a little out of phase," said Conner. "They chose to split away from us and we went to the right side of the course and they went left." The wind shifted in Conner's favor. "We were fortunate," he said. "We played things a little better, or God smiled on us."

If God didn't, a lot of Americans did. After the finish Conner sailed Liberty through a maze of pleasure boats full of cheering spectators. The jam-packed 100-foot excursion vessel East Chop roared up alongside and a crowd of smiling faces rendered a remarkably on-key, rousing "God Bless America."