To hear Iowa's US Senate candidates talk these days, the other man might as well have been elected by voters from another planet. That's how far out each says the other's voting record has been since Iowans first elected him.
In a sense, both Republican incumbent Sen. Roger Jepsen, who seeks a second term, and his Democratic challenger, US Rep. Tom Harkin, have a point. Political analysts say the two men are more radical - on the far right or left wings of their parties - than candidates Iowans traditionally have elected in years past.
Senator Jepsen, a fundamentalist Christian who favors a constitutional amendment banning abortion, argues his opponent is a tax-and-spend candidate who has not supported a strong US defense. In a highly charged, televised debate Oct. 22 which Iowa State University political scientist James Hutter describes as ''a real donnybrook,'' Jepsen said his opponent's pitiful support of Pentagon requests would not allow defense of Hawaii - let alone the US mainland.
And Mr. Harkin, a five-term congressman who favors limits on foreign-made auto parts, no tax hike unless in the context of tax reform, and less US involvement in Central America, argues that Jepsen lacks leadership capability and follows Reagan policies almost blindly.
In some ways the battle is at its fiercest in campaign ads. Jepsen's portray Harkin as a loose spender with a lax attendance record. In one, a farmer supporting Jepsen describes Harkin as a ''slick-talking lawyer'' who in adding no drought relief to his dairy bill is no friend of Iowa's farmers.
Counter ads from Harkin defend his attendance record (citing data to show it is as good or better than Jepsen's) and stress his several votes against raising the cap on the nation's debt ceiling. And Harkin ads describe Jepsen as the big spender, with his support for defense measures, and accuse him of unfairness in his support of social spending cuts.
Jepsen, who stresses morals and family values in his campaigning, appears to have weathered the disclosure last June that he had visited a sex club in 1977. He has stressed that the visit was before he became a ''born again'' Christian. But some Iowa voters still view his congressional immunity dodge of a traffic ticket in Washington, D.C., last year as an example of inexcusable arrogance.
The senator has been running behind in most polls, but about half of Iowa's voters are undecided, and Jepsen has come from behind at the last minute to win before.
Terming the race ''dead even,'' Drake University public administration expert Hugh Winebrenner says it is the most negative, ''mudslinging'' Iowa Senate campaign he can remember.
Last of six pages on regional contests in the Nov. 6 election. Previous pages appeared Sept. 6 and 20, Oct. 11, 23, and 26.