Iowa hasn't reelected an incumbent senator in 18 years, and Republican Sen. Roger W. Jepsen doesn't appear about to break the tradition.

As this Midwestern state's most expensive and nastiest Senate campaign comes to a close, the conservative Jepsen, who won an upset victory in 1978, is trailing five-term Democratic Rep. Tom Harkin by 10 points -- 55 to 45 percent -- in the Iowa Poll published today in The Des Moines Register.

Harkin's lead, his superior field organization and the merely moderate enthusiasm for President Reagan here are among the reasons Jepsen is considered one of the Republicans' most endangered incumbents Tuesday.

Jepsen says he does not believe the polls, and perhaps for good reason. Six years ago, the same Iowa Poll erred in predicting that Jepsen would be defeated by then-Sen. Dick Clark. In 1978, however, interviewing for the poll stopped nearly 10 days before the election.

David Eno, Jepsen's press secretary, called the race "a dead heat," adding, "The momentum is clearly on our side."

To shore up Jepsen, the Republicans this week brought in Reagan, his wife, Nancy, and daughter, Maureen. During the campaign, 30 Republican senators have come to the state on his behalf, and Iowa Rep. Jim Leach, a moderate Republican, filmed an endorsement ad for Jepsen.

Walter F. Mondale and Geraldine A. Ferraro have been in the state regularly, but Harkin has used few other non-Iowa campaigners. However, he is using a heavy dose of radio ads featuring actor Robert Redford and warnings of the dangers of nuclear war by Dr. Helen Caldicott.

The campaign wound down to its final days when the candidates could not agree on rules for their fifth televised debate. Jepsen wanted to use notes, but Harkin objected.

Today was "abortion Sunday" in Iowa, as "pro-life" groups stuck leaflets on car windshields in church parking lots across the state. The technique, used in the past on the Sunday before Election Day, has been instrumental in defeating liberal Democrats in past elections.

But polls indicate that the abortion issue is having less influence this year, especially on Democratic voters, and there are 60,000 more registered Democrats than Republicans in Iowa.

Unlike ex-senators Clark and John Culver, who largely ignored conservative attacks and lost in 1978 and 1980 respectively, Harkin relied heavily on negative advertising.

"Harkin has been less exposed than Culver and Clark were," said Paul Harsted, senior analyst with Peter Hart Research Associates, which has worked for the Harkin campaign this year. "He voted for a form of the balanced-budget amendment and is not tagged 'liberal' as much."

This has been an unusual election in another respect: Harkin has raised as much money -- $2.4 million plus -- as his Republican rival. Contributions from the National Republican Senatorial Committee stopped abruptly last month, and the Jepsen campaign was forced to borrow $40,000 last week.

Whatever the outcome, most Iowans will be happy to see the end of this campaign.

"People are belly fed up with this race," exclaimed Sioux City insurance agent Harland Sopor, a GOP moderate. "It is so adverse to what Iowans consider decent, honest and important about politics."