Here at Herbert Hoover's birthplace, the performance by Iowa's Republican leaders Oct. 18 helped explain why Dick Clark probably will be this state's first Democratic senator ever reelected - and why the mid-term election nationally is shaping up as another exercise in Republican futility.

The tax revolt was supposed to generate Republican revival. But during a long evening of after-dinner speech-making at the Greenview golf club here, the Republican worthies astonishingly said nothing about cutting taxes. Instead, the party's candidates - including Sen. Clark's foe, former Lt. Gov. Roger Jepsen - assailed big government and big spending in language Republicans have been using since Hoover lost to Roosevelt 46 year ago.

If Republicans are not going to talk about tax reduction, Dick Clark certainly isn't. Relieved of such pressure, he shields the doctrinaire liberalism he practices back in Washington with anesthetically dull good-government preachments. He leads Jepsen by 11 percentage points (according to the Iowa poll) though the final outcome may be much closer. Republican Gov. Robert Ray approaches another landslide reelection as a personal triumph unrelated to party or ideology. Overall, Republicanism steadily declines on the Iowa prairies.

That sorely disappoints national Republican strategists, who had salivated over another crack at Clark. An unknown congressional staffer whose election in 1972 even surprised himself, Clark is the most liberal senator ever elected from Iowa. His 1977 liberal voting record, as measured by the Americans for Democratic Action (ADA), was a perfect 18 out of 18.

The National Committee for an Effective Congress (NCEC) and organized labor immediately designated Clark an endangered species, bestowing abundant funds and advice. Clark, who makes up in patience and self-discipline for what he lacks in inspirational fervor, has faithfully followed the advice.

Accordingly, like other campaigning liberals' he calls for a balanced budget some distant day, and claims he no longer is confident of government's ability to solve problems. Copying George McGovern's South Dakota formula, he downgrades liberal ideology and upgrades farm handouts, declaring: "Most of my accomplishments have been in the agricultural area." But there is nothing said about revising that perfect ADA voting record.

Simultaneously, Jepsen, who sounds mostly like an old-fashioned Taft conservative, has been branded a member of the "radical right." He has backed away from earlier stress on Clark's condoning (as Senate African affairs subcommittee chairman) of communist-backed guerrillas in Africa. So, a gunshy Jepsen has dropped plans to spotlight Clark's inexorable opposition to defense spending.

More telling is Jepsen's failure to sound an obsessive tax-reduction theme, which his own precinct polls show top all interests of Iowans. A Jepsen television commercial boosts the Kemp-Roth tax-reduction bill, and he circulates petitions for it. But on the day ending with the West Branch dinner, Jepsen said almost nothing about reducing taxes and lots about reducing government spending.

Consequently, he faces the problems Republican spending.

Consequently, he fa 46 years but that Kemp-Roth was supposed to avoid. Interviewed by a weekly newspaper editor in Corydon, Jepsen was asked how the rural, retired elderly could manage if welfare funds were cut. Instead of giving Rep. Jack Kemp's answer that tax cuts will generate enough revenue to maintain those programs, Jepsen skirted the question with a non-answer.

Iowa Democratic politicians thank their lucky stars that Jepsen has not gone after the Catholic vote by plugging hard for tuition tax credits, opposed by Clark. Instead, Jepsen scatters his shots by still harping on Clark's Panama Canal vote, attacking the Des Moines Register for favoritism and blundering into opposition to federal help for a failing meatpacking plant in Waterloo.

Why don't Republicans focus on the tax revolt? One party operative told us the Democrats had stolen the issue with the recent congressional tax cut, which actually does not even keep up with inflation and Social Security. A better explanation: innate Republican aversion to putting money in people's pockets instead of tightening their belts.

"Maybe Dick Clark will goof up before the election," one Jepsen county chairman told us. Other Republicans pray that anti-abortion votes will be unexpectedly heavy against Clark. A more realistic hope for Jepsen is a small voter turnout combined with superior Republican organization.

It is all hauntingly similar to what Republicans have counted on for infrequent victories in the half-century after Herbert Hoover's defeat: Eisenhower-like non-party figures such as Gov. Ray; Democratic blunders on exotic issues, such as abortion; and mainly, not too many people voting. Instead of coaxing voters to the polls by promising new economic hope through low taxes, the Iowa Republicans pray they stay home.