DES MOINES, DEC. 3 -- A prominent television evangelist led political rallies and church meetings in Iowa this week to promote a conservative Republican presidential campaign that has been emphasizing traditional moral values.

All of which would be ho-hum news in Iowa this year -- except this time, the campaigning minister was not Marion G. (Pat) Robertson. It was the Rev. Tim LaHaye, an influential leader of the Christian right, who came here to endorse Rep. Jack Kemp (R-N.Y.), who is battling hip and thigh with Robertson for support in Iowa's evangelical community.

Along with his wife, Beverly, president of the national conservative group Concerned Women for America, LaHaye said "born-again" Christians should throw their support to Kemp, rather than Robertson, because of the New Yorker's experience in politics and his "ability to reach out beyond the born-again community."

Kemp called the LaHayes' endorsement a significant coup because it demonstrates that Robertson "doesn't have any kind of a monopoly. . . support from evangelical Christian."

Kemp's trumpeting of the endorsement also marks a tacit concession on his part that Robertson's energetic campaigning among evangelicals has become a serious obstacle to the success of the Kemp campaign. As Kemp campaign manager Edward Rollins puts it, "Pat Robertson's constituency is a very important part of the success of a Kemp nomination."

The LaHayes' announcement comes as Iowa political circles are buzzing with debate over the "invisible army" of born-again Christians that Robertson says will carry him to victory in the Iowa caucuses on Feb. 8. Two basic questions have been raised about this army: Does it exist? And, is it really united behind Robertson.

The argument over the evangelical vote is all part of an increasingly visible "expectations" game the Republican contenders are playing here. Robertson's opponents have been emphasizing the evangelist's Iowa strength in the hope that they can create expectations he cannot meet. In recent weeks, top aides to both Vice President Bush and Sen. Robert J. Dole (R-Kan.) have suggested that Robertson might win in Iowa, a prediction that does not appear to be supported by polls.

Robertson helped build expectations about his chances when he said he could turn out 70,000 backers for the caucuses -- a figure that would smash Iowa records. But Robertson's Midwest field director, Marlene Elwell, is working hard to lower expectations. She said today that the 70,000 estimate was "just an idealistic number." She said the Robertson forces "will be very happy if we come in third in Iowa."

One thing Robertson had clearly expected was that he would win the endorsement of the LaHayes. Beverly LaHaye, after all, stood on the platform with Robertson and spoke out for him when he first announced his presidential plans. "He had assumed he'd get our endorsement," Tim LaHaye said here, "because we're dear friends."

But Tim LaHaye said the couple chose to support Kemp in part because they think he has a broader message and a better chance to win. "You can't base a campaign just on born-again Christian values," he said. "There aren't enough of those people to win."

Tim LaHaye also said he was "flabbergasted" when, in the Republican candidates' first televised debate, Robertson presented an economic argument against abortion. The candidate suggested that the decline in the birthrate is costing the country millions of potential workers whose tax payments could help fund the government and its programs. "The only good thing you can say about Pat's answer was that, in the heat of the moment, he must have gotten off on the wrong track," Tim LaHaye said.

For the LaHayes and other evangelicals, the case against abortion is a religious and moral one. Robertson has often explained his antiabortion stand on these grounds, but recently he has been invoking the economic argument instead.

Robertson said today that the Kemp coup will not hurt him in Iowa, because "endorsements don't matter much anyway." He responded brusquely to the LaHayes' suggestion that he can't win, saying, "If you live inside the Beltway in Washington . . . you probably don't know what's going on out here." The LaHayes live in Washington.

Amid all this hullabaloo, meanwhile, opinion polls seem to show that evangelicals in Iowa prefer either Dole or Bush to Kemp and Robertson. An NBC News poll of Iowa Republicans last month showed Dole with a strong lead among those surveyed who identified themselves as born-again. Bush ran second, with Robertson and Kemp both well behind.

That poll suggests that evangelicals here line up essentially like other Republicans.

But the support of Christian right is more important to Kemp and Robertson than to other Republican candidates. Both of them need a born-again base to offset relative weakness in the overall standings.