House Democratic political strategists, mapping plans for the fall congressional elections, believe they have found an unlikely ally in the coming campaign against the Republicans -- the Rev. Jerry Falwell.

Stung by the success of Christian fundamentalist activists such as Falwell in 1984, particularly in southern races, the Democratic Congressional Campaign Committee (DCCC) is planning a counteroffensive this year that its leaders hope will tie the often controversial Falwell to Republican House candidates. The Democrats are gambling that the strategy will hurt GOP candidates embraced by Falwell.

"In 1984, we got surprised a little and thought we needed a defensive strategy," said Martin D. Franks, DCCC executive director. "But there are a lot of places where we can turn to the offensive. Any place where the Moral Majority [the former name of Falwell's organization] gave money, we can say that this person is Jerry Falwell's hand-picked candidate."

In March, House Democrats will send out a nationwide fund-raising letter emphasizing Falwell's "increasing influence in the Republican Party, his success in 1984, his plans for 1986," according to Mark Johnson, the committee's communications director. A 1981 effort to portray the Republicans as captives of New Right spokesmen, including Falwell, was not particularly successful, but Franks said he believes the mood of the country has changed since then.

The committee also plans to produce radio and television commercials tailored for use by Democrats anywhere in the country where Falwell and his organization, recently reorganized under the new name of the Liberty Federation, are backing the GOP.

The commercials "will go after the mere fact that Falwell has endorsed the Republican in the race," said Johnson.

"When Falwell is very active, it offends them [voters]," Johnson said. "His mere involvement is a negative."

Democrats have been encouraged to take this approach not only by their November statewide sweep in Virginia -- Falwell's home base -- but by two recent polls of congressional districts in Texas, now represented by Republicans, in which "born-again Christians" gave Falwell an unfavorable rating.

That Falwell is disliked by many voters is not disputed. His decision last month to submerge the well-known name of Moral Majority into the Liberty Federation was seen as an effort to blunt negative connotations of the Moral Majority label.

Even in Republican polls measuring the standing of a broad assortment of public figures, Falwell "doesn't score particularly high," said Joseph R. Gaylord, executive director of the National Republican Congressional Committee.

The larger question is whether Falwell's standing is an early sign of a more general backlash against conservative religious activism in politics.

Democratic strategists say they are convinced that this is the case and question only whether the reaction will come soon enough and strong enough to be a factor benefiting Democratic candidates in the 1986 congressional elections.

"We're gambling that it will reach critical mass in 1986," Franks said. "It may not until late 1987, but there is no doubt that it is coming."

Gaylord, director of the Republican committee, dismissed the Democratic analysis as wishful thinking.

Noting that Falwell and other Christian fundamentalist leaders have been "very instrumental" in recent Republican successes, Gaylord said, "If the question is, is that whole sort of [religious] urgency in politics falling by the wayside, my answer is I don't think so."

In predicting the collapse of a Republican coalition that includes Falwell as well as younger voters who are "pro-choice" on abortion, he said, the Democrats ignore the broader reasons for GOP gains and their own history of long holding together an equally contrary coalition that included "white southern segregationists and northern blacks."

As for Democratic plans to use Falwell as a wedge to help divide the Republican camp, Gaylord said, "We have found that unless things are in horrendous shape, using a particular target doesn't work. It just is not enough."

But Franks disagreed. Tying the Republicans to Falwell, he said, "is a killer."