The voice boomed across the cornfields at high noon -- a defiant challenge to the conventional wisdom that a liberal must bend to the winds from the right to survive politically in 1980.

Right out in the middle of southern Iowa, speaking to several dozen supporters at a midday barbecue outside that small railroad city, Sen. John C. Culver (D-Iowa) was defending his vote for aid to New York City as a matter of good-farm-belt politics.

"If you don't have an Iowa senator who can talk to the urban senators, who understands the interdependency of our economy, then it's goodbye to the farm," Culver roared. "It may make some people feel good to kick New York around, but I'll tell you, they can kick back."

Only a few hours before, among other supporters at a roadside cafe outside an even smaller farm town, a question about military hardware prompted an impassioned 20-minute oration in support of the now-shelved strategic arms limitation treaty.

And the day before, in a strike-torn Mississippi River town, a radio reporter's question about labor strife led to a flat-out call for enactment of union-backed labor law package, a filibuster-battered bill that many senators have been glad to forget.

Be it a profile in courage or an exercise in folly, the outspoken, toughtalking Culver is determined to slug it out He may have ducked the contest between President Carter and his old friend, Sen. Edward M. Kennedy (D-Mass.), for Iowa's Democratic convention votes, but on issues, he's trimming no sails.

He is betting his political career -- 10 years in the House and six in the Senate -- that he can ride out the storm that is threatening a half-dozen or more liberal Democratic senators this fall by running on his record, rather than away from it.

"When it comes right down to it, John is, well, bullheaded . . . smart, principled, thoughtful but bullheaded," said a friend and admirer. "It could be his salvation or undoing."

Rep. Charles E. Grassley (R-Iowa), Culver's staunchly conservative GOP challenger, is also betting his career in politics -- six years in the House following 16 years in the state legislature -- that Culver is marching resolutely out of step with a majority of Iowans.

"He represents the Old Left," says Grassley, who was benefited, at arm's length, from the New Right movement's campaign against Culver and other Senate Democratic liberals. "He's been for big government, big spending, high taxes and everything else that's brought on inflation and a recession at the same time . . . he's in trouble because history is proving me right and him wrong."

All in all, says Grassley, whose normally serious face breaks into a smile at the thought, "It's a good year to be a Republican and an even better year to be a conservative."

"It's a tough time for progressives," says state Democratic Chairman Ed Campbell, not smiling.

It was two years ago that Iowa shocked the country's liberal establishment -- and lit a fire under Culver -- by unexpectedly tossing out his liberal Democratic colleague, Sen. Dick Clark, in a favor of a relatively unknown Republican conservative, Roger Jepsen.

For a time Culver held his own in the polls. But in the last two surveys by the Iowa Poll, in April and May, he was running nine percentage points behind Grassley. His only consolation was that his supporters appear solid and nearly one out of every four voters is undecided. "Clark's problem was that his support was a mile wide and an inch deep," said one Iowa Democrat. Culver's support, he added, is "more like an inch wide and a mile deep."

Culver's plight was underscored when Grasley, in a victory that was likened by the Des Moines Register to the Mount St. Helens eruption, swept the Republican senatorial primary by a 2-to-1 ratio over a high-spending moderating who had the backing of Iowa Gov. Robert Ray.

Grassley had campaigned hard, returning to Iowa for 57 straight weekends before the June 3 primary. But then Culver, who was not challenged Iowa most weekends for nearly two years, seeking to help revive the Democratic Party after its demoralizing loss of Clark.

Yet a familiar refrain about Culver here is that he's lost touch with Iowa, physically as well as philosophically, even though Culver claims to spend much of his time, even while in Washington, on the state's agricultural, transportation and related problems. The legislative record, ranging from flood-control levies on the Mississippi to soil erosion protections needed by western Iowa, seems to bear him out. But he -- more than Grassley -- suffers from identification with Washington, which he refuses to identify as enemy territory.

His reputation in Washington is to solid that the New Yorker's Elizabeth Drew wrote a series about him, later published as a book entitled "Senator." But recognition in Washington can be a two-edged sword at a time when many people are angry about bureaucrats, regulations and taxes. Said Iowa Republican Chairman Steve Roberts: "It may do well in the Washington salons to be written about in the New Yorker, but it doesn't do much for you in Grundy Center, Iowa."

Here, too, the images and backgrounds of the men themselves -- presenting as sharp a contrast as any set of senatorial combatants in the country -- play an important part.

At 47, his blond hair beginning to recede, the huge, bear-shaped Culver looks like an ex-Marine or perhaps a former college fullback. He is both. But the college was Harvard, and one of his classmates was Kennedy, for whom Culver worked for a year as a legislative assistant.

Culver can poke fun at himself, especiallly at his girth, but he also has a temper of legendary proportions. He is a passionate orator, eloquent but jarring in the contemporary style of media-cool politicians. Culver's family and his wife's family have deep roots in the Midwest, but he is a lawyer (Harvard again) rather than a farmer.

Grassley, 46, tall but trimmer than Culver, slower to smile and slower to anger, is running as a farmer's farmer -- "Grassley from the grassroots" is one of his favorite slogans. He has a 200-acre corn and soybean farm near Waterloo that is now being worked by his son. But he also proudly tells listeners that he once worked on an assembly line and held a union card; he even tells people he once drew unemployment benefits. He is no match for Culver on the stump but has an earnest folksiness that comes across in small crowds. "With Chuck Grassley, what you see is what you get," said the GOP's Roberts.

Culver has taken most of the heat so far, but is now gearing up to try to take the offensive. He has scalded the New Right as a "new species of radical conservatism . . . that seeks to tear down, not build up," urging Iowans to reject what he calls "the hate factories of the East."

Although the National Conservative Political Action Committee, working in conjuction with anti-abortion and other single-issue, right-leaning groups, has targeted Culver for defeat, the attack appears to have left less of a mark in Iowa than in some other states so far. But they were a factor in Clark's 1978 defeat, and the Democrats are wary.

Grassley walks a narrow line on the anti-Culver campaign. Unlike Republican candidates in some states, he urged the NCPAC to stay out of IOWA, but he has not disavowed their message.

Culver is also bearing down on Grassley on two of Grassley's biggest issue: defense and a balanced budget. "Grassley voted five times against the B1 bomber, and now he's flip-flopped and is for it," said Culver, who as a member of the Senate Armed Services Committee helped lead the fight against the controversial bomber. "When he voted against it, it was to cost $30 billion . . . now it would cost $60 billion to build a plane that's obsolete. Now who's the big spender . . . who's strong on defense."

Culver is mounting a similar attack on Grassley's Social Security record, accusing him of viting for higher benefits and against the taxes to finance them, in an effort to undercut Grassley's courting of the older folks' vote.

Some Democrats talk wistfully of a backlash against the lurch to the right. They were at least monentarily encouraged by Republican dissension arising out of recent conservative-controlled state party platform convention that left many moderates angry. Some also claim a "gut feeling" that voters may seek to hedge their bets in the Senate by keeping the Culver-Jepsen balance.

Just as Democratic Chairman Campbell was discussing the GOP's potenial problems with a reporter, a young man wearing an Anderson-for-President button walked into his office, declared himself a "moderately disaffected Republican" and asked for some Culver bumper stickers.

But an Iowa Republican, who is no fan of Grassley's, said he thought the mood of Iowa voters is such that it's beyond Culver's power to change the outcome. "I'd be suprised," the Republican said, "only if the vote is close."

Says Culver: "In politics, 24 hours is an eternity and I still have four months."