Sixteen months ago, Sen. Dick Clark, a liberal Democrat from the supposedly conservative state of Iowa, was riding high. His job approval rating stood at an impressive 70 percent. Farmers, old folks and even Republicans loved him.

His name appeared on the network news with unusual frequency for a freshman senator. And he had a friend in the White House, a Georgia peanut farmer who had gotten the first big break on his road to the presidency in the cornfields of Iowa.

Republicans were dispirited, one GOP official recalls. "We all were sitting back and saying, 'My God, how can we beat that clown.'"

But today Clark is in trouble. His approval rating has plammeted 18 points in the respected Des Moines Register poll. He has become a public enemy in the eyes of many farmers. And Republicans are quietly telling one another that Dick Clark can be beaten.

Almost everyone in both parties agrees on the chief reason why: It is Clark's friend in the White house. Simply put, jimmy Carter has become about as popular in Iowa as a giant corn borer.

Carter seems to have alienated almost every group in the state:farmers, organized labor, small businessmen and even Democrats.

"I've told him what kind of trouble we're in here." Clark said here the other night. "His popularity is at an awfully low point in Iowa. It's going to be gard for him to regain it."

In this traditionally Republican state, the future of Democrat Clark is seen to be tied closely to that of the president. "If the president comes up in the polls, Dick Clark will come up," says David Nagle, a Waterloo attorney and Democratic activist. "If the president goes down, he'll go down."

Carter's popularity has been doing nothing but going down here. It nosedived an astonishing 35 percentage points - from 80 to 45 - in the Register's poll between March 1977 and April 1978.

Clark, 48, is still fovored to win a second Senate term, something no Iowa Democrat has ever accomplished. But party stalwarts are decidedly less confident of the outcome than they were even six months ago.

"It is going to be a rough fall for us," declares state Democratic Chairman Ed Campbell. "Dick Clark has been caught in Jimmy Carter's lack of popularity . . . It is going to be harf for us to stop the herd before it goes off the cliff. We've get to turn it before it's too late."

Republicans will pick Clark's opponent in a June 6 primary that has become a battle between moderate and conservative wings of the party. It has at least symbolic national implications.

The front-runner is former lieutenant governor Roger Jepsen, who is running as "the Right Republican for Iowa." Right means just what it sounds like:Jepsen is making astrong pitch for conservative Republican votes and dollars.

There are those, of course, who believe that almost everyone in the state is a conservative Republican. Granted, Iowa, the state that gave the nation Herbert Hoover, has produied its share or died-in-th-wool conservatives.

But state politics have taken a decidedly 'Democratic turn in the last 15 years. Today the Democrats control the state legislature, a majority of the state's courthouses and four of its sixcongressionl seats. Moreover, the voting record of its two senators, Clark and fellow Democrat John Culver, are among the most liberal in th Senate, and its popular Rebublican governor, Bob Ray, has made much of his reputation by sounding like a Democrat.

Ray's unofficial favorite in the Senate race is state Commerce Commissioner Mourice Van Norstrand, who took to driving a cab at night at few years ago when he found his state salary wasn't enough to send his children to college.

The Republican National Committee, in an extremely controversial move, took sides in the three-candidate primary and funneled $5,000 to Van Nostrand's campaign. But he has been unable to attract much other financial support, adn his campaign appeared to be boring even the mythical old ladies in Dubuque until a week ago when he started lashing out against Jepsen.

He attacked Jepsen's attachment the right wing. He attacked him for hiring George Wallace's former fundraiser, Richard Viguerie. He attacked him for advocating that the Social Security system be turned over to private industry, forcing Jepsen to deny he had ever said it.

"A lot of people apparently don't know it, but I was Barry Goldwater's statewide campaign chairman in 1964," he continued. "I have no diesire to take that hearty little band and mold them into a tight-knit little group who would rather be right than politically effective.

"A lot of people apparently don't know it, but I wa Barry Goldwater's statewide campaign chairman in 1964," he continued. "I wish the world was like that we thought it was until election day. But it was not them and it is not today."

Van Nostrand calls Ray his closest friend. He was urged to run by the governor, and it will be considered a defeat for Ray's brand of moderate Republicanism if he loses.

Jepsen is a skillful nuts-and-bolts politician who built up a solid statewide organization in two successful races for lieutenant governor. On the stump, he looks a central-casting version of an old-time senator, with snow-white hair and three-piece seersucker suits.

He avoids attacking Van Nostrand directly and has an uncanny ability to slip around issues. He reserves his wrath for Clark and his liberal voting record - a record, he says, that "does not reflect the traditional values and more common sense approach that Iowans believe in."

If he should win the Republican nomination, he will give Iowa voters a straightforward ideological choice next November. Clark's strategy, he said in an interview, "will be to paint me as a real right-winner who is against change. But it won't work. I'm a conservative for change. I have a record for my years as leutenant governor and state senator. It's not a record as a right-wing demon with horns."

Clark's election to his first term was a kind legislative aides in Washington dream about. In 1972, he was administrative assistant to then-congressman John Culver, who was thinking about running against the incumbent Rebulican senator, Jack Miller. When Culver decided Miller could'n be beaten, Clark jumped into the race. Nobody gave him a chance.

But he walked the length of the state, talking to hundreds of voters in towns and cities, and he upset Miller.

That walk is still his strongest election suit. "If you ask anyone in Iowa today what they know about Dick Clark, they'll tell you he walked across the states," says the third Rebublican candidate, Joe Bertroche, who uses the hot air balloon as a campaign symbol to dramatize the impact inflation is having on the state.

The image the walk built and Clark's tending to political fences back home were enough to discourage early opponents. Aftr Ray decided to run for another term as governor it looked like Clark would have clear sailing to a secon term.

But that was befor last winter's farm protest, the furor over the Panama canal treaties, which Clark supporter, a series of White House patronage blunders in the state, and Carter's fall from grace among voters

"People expect a lot from Carter here. They thought he'd be another John Kennedy," said democratic chairman Campbell. "They thought he understood our problems because he as a farmer. He disappointed them."

Ironically, just as Clark has suffered from events largely our of his control, he may benefit from them this fail. Wheat and cattle prices are up now. If other farm prices follow, farmers may change their minds about Jimmy carter.

In addition, Clark's service as chairman of African subcommittee of the Senate Foreign Relations Committee, which as led to attacks that he was "more interested in Africa than Iowa," may prove a plus as events continue to heat up on the continent