Questions about the dark side of Pat Robertson's campaign multiplied last Thursday when Rep. Jack Kemp's Rockingham County, N.H., coordinator received a phone bank call posing this question: ''How can you support a man for president who said he is pro-life but allowed his own daughter to have an abortion?''

That statement is pure libel. It repeats accusations that first surfaced in calls and fliers at the 11th hour before the Iowa caucuses. Its source, without doubt, is the Robertson constituency. His actual campaign vigorously denies either inspiring or condoning such gutter tactics.

Whatever its source, the new tenor of the campaign surfaced on the afternoon just before Iowans went to their Monday-night caucuses. Robertson supporters dispatched through the state a crudely written, crudely produced flier.

Addressed from ''Iowa Citzens for America'' to ''Chairmen of Republican Precincts of Iowa,'' the flier said Kemp ''claims he supports Contra aid'' but skipped the vote. It adds Kemp ''claims he opposes abortion. But does he deny that his own family was involved in an abortion?''

In fact, Kemp broke off his Iowa campaign to go Washington to vote for contra aid. In fact, he does deny that any member of his own family was ''involved in an abortion.'' While the Robertson campaign vows no connection with these grass-roots smears, some link is unmistakable in the 11th-hour libel in Iowa contending that Kemp supports pornographic magazines in convenience stores.

Kemp was inaccurately recorded by the Presidential Biblical Scoreboard as being opposed to a ban on such material, thanks to a misunderstanding in an interview with Kemp. That innocent mistake was then incorporated by the Robertson campaign into a skewed rating of major candidates that incorrectly had Kemp as the lowest on the scorecard.

Robertson spokesman Scott Hatch first told us the material distributed in Iowa was assembled by the Illinois state campaign without national approval. Then he said there was no Robertson connection. ''We knew nothing about it,'' he told us. ''Jack has been very consistent on anti-pornography.''

Whether self-starting or not, these attacks fit Robertson's vigorous effort to prove he is the candidate of not merely the religious right but the conservative movement. Up until tomorrow's voting in New Hampshire, Kemp has been the biggest obstacle in his way. The result has been accusations that make recent Bush vs. Dole dueling look like lollipops.

The assault began after Kemp, shocking his own conservative backers, switched sides in Michigan and coalesced with Bush instead of Robertson. But that was an excuse as much as a reason. The enmity was sealed last fall when Kemp traveled on Robertson's campaign plane. Seeking public office for the first time, the religious broadcast tycoon urged the 17-year-veteran congressman to step aside for him.

Since the explosion of their marriage of convenience in Michigan, Kemp has become Robertson's target. They have substantive disagreement on two key points, with Kemp opposing Robertson's support for a balanced-budget amendment and privatized Social Security. But the anti-Kemp campaign is geared to ad hominem questions of competence and character, painting the picture of a conservative poseur. It is the anonymous broadsides that have raised the new level of nastiness which in turn produced last minute grass-roots Iowa defections from Kemp.

Addressing the Conservative Political Action Conference in Washington Friday, Kemp said that even if candidates did not abide by the Republican 11th commandment they should obey the Biblical ninth commandment: ''Thou shalt not bear false witness.''