THE CHRISTIAN POLITICAL activists who helped sweep Ronald Reagan and George Bush to the White House in the 1980s are now threatening to do the same for the Democrats.

Their aim is not to elect Democrats - it is to force the Republican Party into a more culturally conservative mold. But the result could well be an unprecedented off-year congressional victory for the Democrats this fall, and maybe even the election of a new Democratic president in 2000.The defection of conservative Christian groups could be the final severing of the broad-based conservative coalition Reagan put together in 1980 and held together through 1988.

The heart of the Reagan coalition was made up of the same conservatives who nominated Barry Goldwater for president in 1964 and went down with him to overwhelming defeat. But Reagan, unlike Goldwater, was that rarity among Republicans, a man who instinctively knew that to win he had to broaden his base without walking away from his core supporters.

In 1980, therefore, his coalition included not only conservative Republicans but also moderate Republicans, Southern Democrats, blue-collar workers and first-generation and second-generation ethnic voters. The result was a single bloc of fiscal and cultural conservatives from both parties.

After he was elected president in 1988, George Bush, who as Reagan's vice president should have known better, spent the next four years dividing the coalition into the Reaganites and the Bushites. Not surprisingly, he lost his bid for re-election.

In 1994, Newt Gingrich reunited the Reagan coalition behind the Contract With America.

Today, however, Republicans are in danger of losing their grip on the House, thanks to another splintering of the coalition that brought them to power.

Though there are ideological splits over such issues as whether the budget surplus should be used for tax cuts or debt reduction, the most serious division in the party is between the Christian right and the Republican leadership.

Christian leaders, including James Dobson of the "Focus on the Family" radio talk show, Donald Hodel of the Christian Coalition and Gary Bauer of the Family Research Council, are regularly threatening to withdraw their support of the party.

Congressional Republicans, they complain, are not living up to their commitments, which include a constitutional amendment outlawing abortion and the abolition of the National Endowment for the Arts.

Both Hodel and Bauer were high-level Reagan appointees. Hodel was secretary of energy and secretary of the interior, while Bauer served for a time as Reagan's chief domestic adviser. They have not been publicly critical of their former boss for his failure to push for action on these very cultural issues.

Now, however, they and other conservative Christian leaders say they are tired of broken promises. Bauer is threatening, if that is the right word, to run for president. Dobson has been the most vociferous in threatening to end his support of the party. He has not said where he will go, but his choices - start a third party, stay home or join the Democrats - all seem counterproductive.

Dobson and his friends say they have waited far too long for the promised progress on their causes. Yet progress, even for worthy causes, is made slowly.

If they walk away from the Republicans because they think they are being badly treated, the religious rightists will surely find, when they have turned the Congress and the presidency back to the Democrats, that they ain't seen nothing yet.