From one end of Iowa to the other, an army of the faithful marched to church Sunday morning, singing a requiem for a liberal.

There were other potent factors at work, but by early yesterday, the state's well-organized right-to-life army had made its contribution to the defeat of Sen. Dick Clark.

Clark, a landslide winner in 1972 when nobody thought he could win, was defeated by Republican Roger Jepsen, a conservative businessman from Davenport and the state's former lieutenant governor.

Late returns gave Jepsen 51 percent of the Iowa vote and relegated Clark to a status shared by such other Senate Democratic liberals as Hathaway of Maine, Haskell of Colorado, McIntyre of New Hampshire and Anderson of Minnesota.

Even in a year that saw the fullblown emergence of the "single-issue" lobby, this was not supposed to happen - at least in Iowa.

Despite a bitter campaign that drew national attention, Clark, according to the usually reliable Iowa Poll of the Des Moines Register, had a comfortable 10-point edge as recently as Sunday.

But, as it turned out, Republicans did well statewide, the turnout was light, Clark had been battered on other issues - foreign policy, gun-control, tuition tax credits, his image - and the poll was wrong.

Abortion was one of the issues.

The state's anti-abortion forces took vocal exception to Clark's position for liberalization of abortion laws and made him a target for defeat.

Their final salvo was fired Sunday morning at churches across the state, when they distributed 300,000 brochures urging Clark's defeat for his refusal to support a right-to-life amendment to the Constitution.

The campaign was directed by the Pro-Life Action Council, which was formed earlier this year to attempt to influence elections.

The council warmed up in the state Democratic primary, taking partial credit for the defeat of a pro-abortion candidate for the nomination for lieutenant governor.

"That was the basis for what we did in the Senate race," said Des Moines attorney Robert Dopf, secretary-treasurer of the council.

"We built for the Senate race and Clark was our target . . . We had met with him for two years, but he said he was voting his conscience on this. We said we had no alternative but to oppose him."

The council endorsed Jepsen and then, winding up its campaign, conducted the leaflet exercise outside the churches on Sunday.

"There is not much argument from anyone about the impact we have had," Doph said. "The handwriting is on the wall for more than Senator Clark . . . Politicians are going to have to deal with this issue."

"The network we have here is tremendous. We see it as a foundation for the 1980 presidential campaigns. We are very optimistic."

The Iowans also claim some share of credit for the defeat of another liberal asporant - Minnesota Rep. Donald Fraser who was supset earlier in his quest for the Democratic senatorial nomination there.

Minnesota pro-life forces used the Iowa brochure as part of their effort against Fraser.

Not coincidentally, perhaps, Clark and Fraser shared similar views on human rights and against a policy of U.S. intervention in Africa.

Clark, as chairman of the Senate's African affairs subcommittee, was instrumental in halting covert U.S. involvement in Angola. His influence on U.S. policy has been substantially more than a first-term senator might expect to have.

That, too, might have figured in his defeat Tuesday. Jepsen had accused him of helping Russia and Cuba with an appeasement policy.

"When Angola came along, he became more interested in foreign affairs. It made me think he was building a solid record as a spokesman on foreign affairs, but his Iowa image was slipping," said John Murray, a Republican state senator from Ames.

"People felt jilted, estranged from someone who had been their walking neighbor. He was too busy for their problems," Murray said. "He suffered from Fulbright's disease."