SIOUX FALLS, S.D. -- Lillian Bingham looked up from her checkbook, a puzzled expression creasing her kind, round face. "Politics?" she said, responding to a question. "Have I ever been involved in politics? Why, no. Not unless you count voting for Franklin D. Roosevelt."

And yet here she was, sitting in a crowded pew at the First Assembly of God church on the edge of the cornfields here, writing a $100 check to a political candidate. "Well, this man is different," Bingham explained. "I feel like our country needs a leader who is a strong moral leader. And Pat Robertson is a moral man."

Bingham's comments -- and her financial contribution -- are being replicated time and again in churches and meeting halls around the country these days. Thousands of evangelical Christians are turning out to hear, cheer, and volunteer for Marion G. (Pat) Robertson, the television evangelist.

The first political signs of that fervor were on display last weekend, when Robertson easily won a straw poll in Iowa, shocking the front-running GOP candidates Vice President Bush and Senate Minority Leader Robert J. Dole (R-Kan.), whose organizations had spent extensive time and money trying to win.

Judging from a three-state tour with the candidate this summer, the people who show up at a Pat Robertson rally generally pay more attention to the pulpit than to politics. They can recite passages from Leviticus verbatim but cannot say which party their governor belongs to. Their lapel pins say "I found it!" or "Praise the Lord!"; their bumper stickers quote scripture.

The evangelical passion surrounding Robertson in this stage of his candidacy gives him an instant political base. Robertson is not attempting to win the backing of traditional Republican activists as well during the race for the nomination. He suggested early this summer that the evangelical movement alone could win him the nomination.

"People keep asking me when I'm going to expand my base," he said on a talk show in Denver in June. "But I don't have to broaden my base. My political base is 70 million people . . . the 70 million evangelical Christians in this country."

This is not to suggest that Robertson has a political hammerlock on the evangelical community, or that any of his Republican rivals is willing to concede the so-called "Christian right" to Robertson in the 1988 primaries.

All Republican hopefuls have been pursuing the evangelical vote and polls indicate that all GOP candidates have advocates among that group.

Even if Robertson does win strong support from the churchgoers he is pulling into politics, that alone would probably not spell victory in many Republican primaries. "Robertson has already proven that the evangelical movement can help in caucus states," said Washington political analyst Kevin Phillips. "That's where you just get a big bunch of committed people and bus them to the caucuses, and you can win. But in a primary election state you need something broader. And I haven't seen a poll in any primary state that gives Robertson more than 10 percent {support} right now."

But at this relatively early stage, the Christian connection is providing Robertson with a priceless body of committed followers, volunteers, and contributors. In Iowa, Robertson supporters, for example, used the signatures of over 30,000 state residents who signed his petitions of support to mine for volunteers and to produce the 1,293 straw poll votes that gave him that victory.

"We have wanted a moral government to lead this nation," says Robert Scheufler, an insurance agent from Parker, Colo. describing his backing for Robertson. "But what we couldn't stomach was that the only route to government is through politics. And now Pat Robertson is getting us into politics."

Other than meeting with officials of his local school system, Scheufler says he has never been politically active. He heard about Robertson this spring at a weekly prayer breakfast with other businessmen. He attended the first $100-per-plate dinner of his life when the candidate came to town. Now he plans to invite a group of neighbors to his home, show them a videotape of a Robertson speech, and thus recruit them to the cause.

The Robertson fans are willing to give money to this political crusade. Although Robertson is not issuing campaign finance reports, he says he has taken in $10 million in campaign contributions, putting him on a par with Bush and far ahead of all the others.

Robertson's aides say their recruits are willing to study and learn how to work in the political system. "Our people are as green as grass," says Lee Coppock, a Robertson campaign staffer in Florida. "If 10 percent of them have ever been involved in politics before I'd be amazed. But they want to learn everything about the process so they can get down to work."

At the Assembly of God church here in Sioux Falls, about 400 fans cheered loudly when the candidate spoke in the sanctuary. Later they crowded around him in the social hall asking how they could help him get to the White House.

"An awful lot of these people are going to end up working in this campaign," predicted Ron Raynor, a Sioux Falls engineer from the baby-boom generation.