In an article about Sen. Charles E. Grassley (R-Iowa) Monday, four words were inadvertently dropped from a quote in the last sentence concerning Grassley's defeat of John C. Culver and the perception that Culver was out of touch with the people of Iowa. The sentence should read: "He hit a nerve because some people thought there might have been a grain of truth to it."

There are a number of political truisms in Washington and even a fool knows this one: Don't cross President Reagan, who is riding high on his landslide reelection. It's really risky.

So, who is this fool who keeps crossing Reagan, insisting on an across-the-board freeze on the federal budget that includes the Defense Department along with everything else, earning him threats of presidential retribution when he runs for reelection next year?

He is Sen. Charles E. Grassley (R-Iowa), and on first sighting he looks like the sort of Central Casting character who comes in from the sticks and gets his clock cleaned in the big city. He has big, farmer's hands, cracked and stained from decades of manual labor, and talks with a nasal, almost Gomer Pyle-type voice. He says "gosh" and "golly" a lot.

He's been known to mangle the King's English and often sits in committee hearings blinking and looking as if he just fell off a turnip wagon. He's the first person to whom a con man would try to sell the Washington Monument. But Grassley is the sort of rube who winds up taking city slickers to the cleaners instead of the other way around. His act plays so well in Iowa that he has become the state's dominant political figure and is well on his way to becoming a folk hero.

Many Republicans are fearful that their party is going to take a beating in the Midwest next year because of the desperate state of the farm economy. But Grassley is considered such a shoo-in for reelection -- his job approval rating is at about 70 percent and has probably climbed in recent days -- that Democrats are having trouble finding a candidate to oppose him.

"You couldn't beat him with a club next year," said James S. Flansburg, editor of the Des Moines Register's editorial page. "He follows his own agenda, picks his spots, learns about an issue like defense spending and makes his mark on it."

Grassley was the first to propose the across-the-board budget freeze, and his mark on defense spending is considerable, according to Rep. Thomas J. Tauke (R-Iowa).

"He has shifted the focus of the debate on the budget and has turned the momentum on defense spending," Tauke said. "Having a conservative champion a freeze on defense spending has made it politically acceptable for other conservatives to oppose growth in defense spending."

Many on Capitol Hill, including Grassley, now say they think that a freeze is a possible solution to the disagreements on the budget.

"We'll get close to a freeze, with Gray behind it," Grassley said last week, referring to House Budget Committee Chairman William H. Gray III (D-Pa.). "We're in sight of it. There has to be a control on defense spending particularly when you can't get a few crumbs from the table for agriculture."

Grassley took on the defense spending issue in an attempt to get federal spending under control. He was not so parsimonious in 1979, however, when he backed a 7 percent increase in grain price supports.

His stance on defense spending also has won him political support from many liberals and independents. Far from being hurt by his face-offs with the Reagan White House, Grassley is getting an enormous boost from them.

The latest run-in was a few days before the Senate vote on the MX missile last week when an underling of political director Edward J. Rollins indicated that the president will campaign and raise money next year for friends who support him on issues like the MX and other defense spending. Grassley was holding out until the Air Force gave him the data on the missile that he had requested.

Grassley responded immediately. He described for reporters a Rollins campaign swing through Iowa last fall while being exasperated that Grassley was trying to get then-Attorney General William French Smith cited for contempt of Congress for not helping more in investigating fraudulent military contracting practices. Grassley said Rollins attacked his positions, using profanity, when talking with one of the senator's supporters.

Rollins denies it.

"I don't say things like that," he said. "We've got some tough votes ahead. And those who help us, we'll take care of. There's no retribution, but no one has a right to a presidential fund-raiser."

Grassley did not get the information he sought on the MX and voted against it, but he is so popular at home that the White House threats are regarded there as attempts to scare the other 21 Republican senators up for reelection next year. Some think the fix was in between Rollins and Grassley because there is nothing the White House could do that would help Grassley more.

"I like the president, but my job is to work with him, not for him and there's a difference," Grassley said. "I didn't pick a fight. I'm just reacting."

Grassley's combative response to people who try to pressure him is to tell them to stick it in their ear. It is just one of many things his constituents like about him.

Despite the apparent differences in their styles, Grassley and Reagan have a lot in common. Both have acute political instincts and both inspire such confidence in their integrity and decency that most of their actions are viewed as being rooted in honest conviction, even by people who disagree with them.

"Chuck comes across like former Republican governor Bob Ray," Republican national commiteeman John McDonald said last week, referring to the most recent giant of Iowa politics. He gave Grassley the ultimate political compliment: "He will do what he can do without political considerations." That may be, but Grassley enjoys the confluence of principle and good politics. One nonstylistic difference between him and Reagan is that he is popular in Iowa and Reagan is not. Reagan's most recent disapproval rating was 49 percent. His approval rating was 42 percent. It may be lower now because of his veto of the farm-credit relief legislation.

Projections by Iowa State University farm specialists are that 15 percent of Iowa's farmers will go bankrupt in the next three years.

Opposing the president on increasing defense spending also is popular in Iowa, which ranks 38th in defense contract dollars. The Des Moines Register responded to the White House threat on the MX vote with a front-page cartoon showing White House spokesman Larry Speakes saying, "And you can forget about the president campaigning for you in '86!" with Grassley responding, "Could I have that in writing?"

Grassley enjoys a generally admiring press in Iowa. He frequently is compared with Jimmy Stewart in the old Frank Capra movie, "Mr. Smith Goes to Washington," about a neophyte who goes to Congress to do good. "Grassley Withstands White House Pressure" was the headline on the Quad City Times story on the MX vote.

The root of Grassley's political strength is that Iowans view him as one of them, which he is. He is an old-fashioned Midwest fiscal conservative and a teetotaling Baptist.

His wife and family live on a farm in New Hartford so his youngest son can play high school basketball there, and Grassley goes home every weekend. He gets angry if he hears that someone from Iowa has been in his Washington office and he did not get to meet them.

"One thing I have going for me is that I haven't waited until the fifth or sixth year of his Senate term to campaign or to establish my independence," Grassley said. "I try to get into every one of the 99 counties to speak and meet with people at least once a year."

When Larry Flynt, publisher of Hustler, sent free subscriptions of the magazine to members of Congress, many responded by self-righteously denouncing Flynt in speeches on the floor. Grassley sent Flynt a letter that read:

"Dear Larry: Since you have sent me a slice of your mind, I'd like to send you a slice of mine. You will shortly receive your first installment of an annual subscription to 'Christianity Today . . . . ' Sincerely, Chuck Grassley."

Grassley ended an interview with the Register by going home to change clothes and move a pickup load of hogs from one piece of land to another. He volunteered to serve the jail sentence for Wayne Cryts, a Missouri farmer who was held in contempt of court for "liberating" 33,000 bushels of soybeans that he had in a bankrupt grain elevator whose assets had been frozen. Grassley introduced legislation to give farmers relief in cases of bankrupt elevators.

For 16 years, while going to the University of Northern Iowa in Cedar Falls and serving in the state legislature, he was a sheet metal worker in Waterloo and a member of the machinists' union.

He was elected to the Iowa legislature in 1958 and to the U.S. House in 1974, succeeding H.R. Gross. He modeled himself after Gross, a dogged fiscal conservative, and after coming to Washington continued Gross' practice of submitting a bill, "HR 144," requiring that the federal budget be balanced.

"My boss when he worked as a machinist would give me a leave of absence so I could go to the state legislative sessions, which met every other year then," he recalled. "If he hadn't, I couldn't have served, I'd have had to work to support my family, and I wouldn't be in the U.S. Senate today."

"Every Monday night the Iowa Federation of Labor has a beer bust, a sort of hospitality reception," said one veteran political observer. "Grassley showed up about three weeks ago. He didn't drink any beer, and he's never going to get their endorsement. But he gets labor votes, and he defuses a lot of natural animosity. You never saw former Republican senator Roger W. Jepsen there. He hated them and vice versa."

When Grassley defeated then-Sen. John C. Culver (D-Iowa) in 1980, winning the highest number of votes in a Senate race, the jokes were that the state's senators were "Tweedle-dumb and Tweedle-dumber" and "What state has a senator dumber than Roger Jepsen? Iowa."

But Grassley has long since put that to rest. And some observers say they think that the contrast between them contributed to Jepsen's defeat last November.

"One of his greatest assets is that he is consistently underestimated by his opponents," Tauke said.

Grassley has a master's degree from the University of Northern Iowa and is a dissertation away from a PhD in political science at the University of Iowa.

He wrote a Wall Street Journal article advocating a spending freeze in which he said that defense has become "the nation's largest entitlement program and has nursed a new generation of welfare queens: the defense industry."

In a letter to the governors advocating his defense freeze, he sneered at Reagan's proposal of an overall freeze with cuts in domestic programs and increases in defense spending as "only half frozen, like a 7-Eleven Slurpee."

Even his opponents admire his political instincts.

"He's very shrewd, with good gut-level instincts," said Lyn Cutler, vice chairman of the Democratic National Committee and an unsuccessful candidate for Grassley's House seat in 1980 and 1982. "You don't beat someone as good and smart and terrific as John Culver without using all your weapons, and he hammered to death the idea that John was not an Iowan and didn't care about the folks there. He hit a nerve because there might have been a grain of truth to it.