Nothing, not even divine intervention, could have saved the Rev. Jerry Falwell yesterday as he sat on a short gangplank and contemplated the icy blue water below.

He looked skyward, hands upraised, smiled a defiant smile, and . . . SPLASH!

The leader of the Moral Majority hit bottom in the dunking tank--much to the delight of about 2,500 celebrants yesterday at conservative, direct-mail wizard Richard Viguerie's second annual American Independence picnic.

No one was happier than Mary Alice Gattis, 67, whose strong right-handed toss of the softball had sent Falwell, fully dressed, into the drink.

Gattis, a member of the American Conservative Union, eagerly confessed, "I used to play baseball, but my God, that was a long time ago." Then she demanded, "I want three more shots!"

As if Gattis wasn't enough for Falwell, Viguerie himself took the second turn on the pitcher's mound, a determined gleam in his eye.

Viguerie missed on all three chances and then ran forward to punch the dunk button with his hand as the preacher yelled "Foul! Foul!"

Standing up to his chest in water, as he did for much of the next half hour, Falwell laughed, "Norman Lear has wanted me like this for years."

No one seemed to feel sorry for the soggy Baptist leader. He may have been the coolest person at the picnic, held at the sprawling Turkey Run park in McLean. The sweaty, boisterous event raised money for the Claude Moore Colonial Farm at Turkey Run and Children's Hospital in Washington.

Viguerie estimated that ticket sales would raise $10,000 for each of the recipients and said his organization had chipped in $25,000 to pay for the hamburgers, hot dogs, country-western bands and other attractions of the five-hour affair.

A potential spat among conservative allies over where the money would go was averted at the last minute, when Olga Fairfax of Methodists for Life called off a threatened picket line. Fairfax last week criticized Viguerie and Falwell for raising money for Children's Hospital, which successfully challenged White House-backed regulations on care for severely handicapped newborns--the so-called Baby Doe rules.

"We had a good exchange of views," said Viguerie. "We all make mistakes, and we think Children's Hospital made a mistake. It's still a wonderful hospital."

Fairfax came late to the picnic and said she had left 300 leaflets on windshields in the parking lot. "This gives people something to read on the way home, whereas the picket they would see only on the way in," she said.

On the broiling fields of competition, meanwhile, a slew of conservative merrymakers and a handful of bashful liberals seemed unconcerned about serious partisan disputes.

Viguerie's two daughters led the winning squad in a relay race dubbed "Balance the Budget." Participants scurried back and forth with actual four-inch-thick fiscal year '84 budget books on their heads, while onlookers shouted, "You're doing better than Reagan!" and "Someone show the president how to do this!"

The "Plug the Leak" contest involved barrels and corks and a bucket brigade, but it quickly degenerated into a free-form water fight with no clear victor. White House aide Faith Ryan Whittlesey ended up cheerfully drenched, admitting, "Obviously, I don't know how to play this game."

Publicist Vic Kamber, usually seen in Democratic circles, vowed he would prove that "liberals can win, even here." Minutes later, he emerged as wet as anyone else and claimed a triumph.

By midafternoon, the green had been transformed into a Revolutionary War battlefield, where two convincingly constumed Colonial platoons took on a badly outnumbered British contingent.

The redcoats advanced bravely, but fell into a crafty rebel trap, complete with hidden cannon. Once the outcome seemed clear, the several American "dead" sat up to have a drink and enjoy the final charge with the rest of the spectators.

"I just volunteered to die," explained freedom fighter Rick Edwards with the casual gallantry of a military hero. "When you feel like dying, you drop dead. It's part of this business."

The hundreds of children trying to keep up with this combination history lesson-carnival raced from puppet show to pony ride to hot-dog stand, much to the amazement of wilted parents.

"It's a chance for conservative progeny to meet and get to know each other," said Howard Phillips, head of Conservative Caucus. "I brought five to add to the mix."

Ryan Viguerie, the host's 9-year-old son, kept his cool under a one-man concession tent, where he sold chocolate-chip cookies for 25 cents each, with a complimentary cup of water. His proud father explained that the boy "seems to be following in daddy's footsteps, always around the office asking questions."

Ryan conceded that he had his eye on the family business, commenting, "It looks pretty easy, just mailing out letters."