It was bad enough, Republicans here thought, when their Senate candidate defended apartheid - but why did he have to do it in the state's liberal city?

And why did Roger Jepsen have to suggest the possibility of shifting Social Security to the private sector? And why did he have to become involved in a zany scheme to stock the offices of the state Commission on the Blind with a secret arsenal of automatic rifles?

Iowa's Republicans aren't sure of the cause of these political events, but they are glumly aware of the effect: Jepsen has probably fumbled away the GPO's chance to win back the Senate seat that Dick Clark snared for the Democrats in an upset victory in 1972.

"It's gone a lot easier than we expected," the portly Clark observed placidly last week during a break in his calm campaign for a second term. "Their campaign hasn't seemed to make any headway."

Outside the Senate race, election day bodes fairly well for Republicans here. The moderate GOP governor, Robert Ray seems to be cruising toward a big victory in his campaign fot the fifth term, and the party's conservative candidate for lieutenant governor, Terry Branstead, is leading in his separate race.

The Republicans should hold their two House seats, and they have a fair chance of retiring Democratic ReP. Michael Blouin to win a third. (Three other Democratic incumbents have safe leads.)

And some GOP optimists still hold out hope for Jepsen in the Senate race. The silver-haired conservative stood 11 points behind Clark in a Des Moines Register poll taken the first week in October. Republicans note that Clark was even further behind in the October poll in 1972, the year he won the Senate seat from Republican Jack Miller. And Miller, in 1968, won in November after trailing by a dozen points in the October poll.

Both of those upsets, however, involved last-minute revelations that badly scarred the leaders. Jepsen, having unleashed all his ammunition against Clark already, seems unlikely to come up with a killer issue now.

Jepsen's bumbling has been a harsh blow to Republican leaders. Six months ago, the party thought it had Dick Clark's Senate seat in its hip pocket.

No Democratic senator from Iowa has ever won a second term, and few politicians at any level here have survived a liberal, pro-labor voting record like the one Clark has compiled. The senator's support of most Carter administration programs was a detriment, too, because of the president's unpopularity here.

Clark responded to these problems the same way he responds to everything else - calmly. He came home every week and toured the state speaking on such scintillating topics as efficiency in government and lobby regulation.

Even now, in the campaign's final fortnight, the average Clark speech could be bottled as a cure for insomnia. But the low-key approach apparently conveys the image of a responsible, knowledgable senator - not the wide-eyed liberal the Republicans have been complaining about.

jepsen's campaign, meanwhile, has been moving at top speed - backward.

About a month after Jepsen defeated a moderate opponent in a Senate primary, the media picked up a wild story about a former director of the state Commission for the Blind who had stocked the commission's offices with guns and ammunition, presumably to defend against riots. Jepsen, a close friend of the official responsibile, was dragged into the controversy and lost a month's time answering questions about it.

Jepsen spent a few more weeks retreating from his suggestion that private insurance companies might do a better job than the Social Security Administration.

The Republicans thought Clark's involvement in African issues - he chairs the Senate's African affairs subcommittee - would be a hot campaign issue. A South African trade minister touring Iowa last spring criticized Clark for siding with terrorists on the continent.

But then the University of Iowa newspaper reported that Jepsen, in a visit to the campus at Iowa City, a liberal stronghold, spoke sympathetically about the racial policies of the South African government. In the resulting to-do, Jepsen toned down his remarks on Africa.

Democrats, meanwhile, think Clark's African connection is an asset. At nearly every appearance, the senator is introduced as "the man who stopped President Carter from sending U.S. troops into Angola," and Clark never misses a chance to bring up his Senate resolution that prohibited introduction of American forces in the Angola civil war.

In the past few weeks Jepsen has struck on a sensitive issue, however, by lambasting Clark for supporting the Panama Canal treaties. "There's still feeling about it here," Clark says, and he was surprised that Jepsen did not use it earlier. Conservative leaders, who put pressure on Jepsen to start talking about the canal, say it could have been a major GOP plus if Jepsen had not waited until the last month of the campaign to bring it up.

The Democrats' fear is that the canal and other emotional issues, such as use of federal funds for abortions (Clark opposed an outright ban) and the tuition tax credit (Clark opposed it), could bring out hordes of anti-Clark votes. That worry is exacerbated by predictions that most voters will stay home on election day.

Public officials here deplore the electorate's obvious apathy toward the election, but a recent trip with Gov. Ray may suggest why the people of Iowa are indifferent.

On a sparkling Indian summer day, Ray went on a campaign swing through Montgomery County, a rich patch of land in the state's southeastern quadrant.

It is a beautiful piece of America - and prosperous as well. The county unemployment rate is 1.8 percent. Cattle feeders and pork producers are enjoying high prices. Farmers are harvesting huge crops of corn and soybeans that should more than offset their losses from two years of drought.

Ray stopped at the ornate red stone courthouse in Red Oak, the county seat, where the Republican Women's Club was finally ready to dedicate a project it had undertaken tow years ago to celebrate the bicentennial.

One hundred fifty happy people cheered when the governor entered the courthouse. They broke into spontaneous applause when he unveiled the project - a giant quilt depicting all fifty states, with autograph of each governor embroidered in blue thread. And when Opel Ingram, of Red Oak, plunked a chord on the courthouse's ancient piano, the whole assemblage gave forth a proud, soaring chorus of "God Bless America."

It was easy for a visitor to see why the people of Iowa think their lives will be all right no matter who wins the elections here on Nov. 7.