Iowa City, Iowa — The man conservative Iowa US Sen. Roger Jepsen (R) terms ''too liberal'' for Iowa finishes an impassioned plea on the need for higher-quality education and hones in on what he calls the priority issue of the campaign. Before a large audience in the University of Iowa's Student Union, US Rep. Tom Harkin (D), trying to unseat Mr. Jepsen, insists the only ''sane'' answer to the arms race is to negotiate a mutually verifiable nuclear weapons freeze with the Soviet Union.
The students respond with loud clapping, cheers, and chants of ''Har-kin, Har-kin.''
Later that day at nearby Grinnell College, Mr. Harkin, a staunch opponent of the MX-missile, accuses Senator Jepsen of serving as a ''rubber stamp for every gold-plated Tinkertoy the generals want.'' As an enthusiastic audience again cheers him on, Harkin insists Iowa needs a senator who knows how to say ''no'' to the Pentagon.
The Democrats consider a Harkin win here in corn, hog, and Bible-belt country crucial in their efforts to regain majority control of the US Senate. Although the GOP currently holds both Senate seats and the governorship here, independent Iowa voters have not reelected an incumbent senator since 1966. And Democrats have been hoping that farmer disenchantment with Republican policies would work to their advantage.
''Both parties see the farmers as the swing vote here,'' confirms Iowa Democratic Party spokesman Barry Piatt. ''They tend to vote Republican unless something goes wrong - and it did. We've been losing 40 to 50 family farms a week through foreclosures for about two years.''
But Democrats here know the job of winning back the Senate seat once held by Democrat Dick Clark is looking tougher all the time.
For much of the last six months Congressman Harkin has been running well ahead (by 17 points in May) of his GOP opponent in the Des Moines Register's Iowa Poll. But in September Jepsen - a fundamentalist Christian who is against abortion and gun control and favors tuition tax credits, vocal prayer in the schools, and most Reagan Administration economic and defense policies - moved into the lead by nine points.
The Democratic Senatorial Campaign Committee this week challenged the mid-September sampling and insisted that Harkin retains a lead. They do concede , however, that Jepsen ''has gained some ground.''
The strongest support for the dignified-looking, white-haired Republican, according to the poll, lies among voters over 65 and under 24 and those living in rural areas.
Each candidate insists he is a moderate and that it is the other man who is at the far edge of his party. But they agree that voters have a clear choice.
Harkin says Jepsen's agenda is that of the ''right wing and Moral Majority.'' Jepsen says Harkin may talk like a conservative but is ''out of step'' with Iowans as a big spender whose votes have helped increase the deficit.
''It looks as if Jepsen's main theme - that Harkin is too liberal for Iowa - is getting across, although I'm not sure it will hold up till the end,'' says Cornell College political scientist David Loebsack.
''Harkin would probably like to see that issue go away,'' notes former state Democratic Committee chairman Ed Campbell. But the same charges, coupled with stands against abortion by the Republicans making them, were largely responsible for Democratic defeats before - that of former Senator Clark in 1978 and former Sen. John Culver in 1980 - says Mr. Campbell.''Iowa is still a populist, middle-of-the-road Republican state; it's not liberal.''
Although a Roman Catholic and personally opposed to abortion, Harkin does not favor a constitutional amendment to ban it. When asked by a reporter about Jepsen's charge that he is out of step with Iowa, Harkin, a lawyer and former Navy jet pilot, stresses that his life style is conservative and that he has been reelected to Congress five times by strong margins in one of Iowa's most conservative districts.
Harkin insists that the issue for voters is not so much ideology as leadership and judgment. The Democrat's campaign slogan - ''A Senator Iowans Can be Proud of'' - predates the disclosure last summer that Senator Jepsen in 1977 applied (in what he has since said was a moment of ''weakness'') for membership in a health club offering nude modeling which was later closed down by police as a brothel. It was before Jepsen's run for the Senate and before he became a born-again Christian.
Polls suggest few voters are inclined to hold that episode against him. More bothersome to many voters was Jepsen's avoidance of a traffic ticket on grounds of congressional immunity when stopped for driving alone in the fast-moving car pool lane of a highway in Washington.
In campaigning, Harkin mentions the latter but never the former incident. And he frequently points to Jepsen's last-minute 1981 vote switch on sending AWACS planes to Saudi Arabia as an example of what he sees as Jepsen's tendency to follow Reagan's lead at any cost. Jepsen first said his tie-breaking shift in favor of the sale was based on classified information. Later he conceded that politics (including intense White House lobbying) and Reagan's freedom to shape his own foreign policy were the key elements in his decision.
Harkin, a strong supporter of Israel, led the House fight to block the AWACS sale.
Political analysts say if Harkin can keep the focus on Jepsen himself, the Democrats could win. The Democratic candidate's only public reference to Jepsen's mistakes is a bid to voters to leave behind the ''old embarrassments'' by voting for him.
Republicans insist they can win if they can shift voter attention from Harkin , an attractive campaigner, to his Washington performance. ''If we can make Harkin's voting record the issue, we'll win,''says Iowa Republican State Central Committee chairman Rolf Craft. Jepsen campaign press secretary David Eno says: ''It's an ultraliberal, if not a radical, voting record.''