NEW HARTFORD, Iowa

Hoisting his corn-fed hulk over the fence and into the sty, the farmer walks among his rooting hogs. The black-and-white-belted Yorkshires, fat and getting fatter, eat in gluttonous zeal the best grist growable on this 240-acre farm in the rural outback of northeast Iowa. The farmer -- 51, black-haired and wearing a mud-splattered shirt -- stands next to a trough and pats his hogs. They snort, squeal and scurry in the muck. But he has them under control. This is a man who understands hog psychology.

He ought to. The farmer is Charles Grassley, Iowa's senior senator and a conservative Republican who in the past two years has been going into the sty of military excess where the fattest breed on earth -- the money hog -- feeds at the Pentagon trough, the world's deepest. Grassley, a member of the Senate Budget Committee, has been the most vocal Republican in Washington to call for a freeze on military spending.

Last month, he wrote in The Des Moines Register and The Wall Street Journal that the Pentagon's budget has "become the nation's largest entitlement program, and has nursed a new genera- tion of welfare queens: the defense industry."

Such language -- which is as close to a barnyard epithet as the evangelical Baptist will get -- is not a sudden outburst. Grassley has been developing in a gradual germination. He voted 15 times in favor of the MX missile, standing as tall as an Iowa cornstalk when it came to loyalty to Ronald Reagan. Then, in June 1984, he changed his mind and began voting against the MX.

What happened? He began studying the Pentagon's procurement policies and learned that military contractors were routinely putting a move on the public. Last month, he explained his anti-MX votes: "I discovered from Air Force documents that work-to-date by the 14 associate contractors for the MX was taking up to 17 times as many direct labor hours as the contractors' own standards determined it should have taken . . . The average factory efficiency rate of those 14 contractors . . . was 48 percent. In other words, 48 percent of the taxpayers' dollars were funding efficiency, and 52 percent were funding inefficiency. We paid for in-house work for 2.1 equivalent units, on average, and got only one."

In New Hartford, the grass-roots Grassley jokes easily about how his militancy is being perceived as a drift to the left. He tells of his conservative right-of-right brother on a farm down the road who thinks the senator is something of a pinko. In fact, Grassley voted 78 percent of the time with Reagan in the past two years. That is down from about 85 percent from a previous period, a drop that signifies apostasy only to the fanatical wing of the New Right.

The Old Right is alarmed for other reasons. Sen. Barry Goldwater wrote a 2,000-word reply to Grassley's profreeze articles. What Washington sophisticates of the left were snickering when Grassley came to the Senate in 1980 -- the guy's a yokel, an airhead -- Goldwater was suggesting now. The Arizonan lectured Iowans that their senator "does his state and our nation a disservice when he passes off his simplistic, self-serving advocacy as reasoned analysis."

Goldwater sought to bomb Grassley's thinking back to the Stone Age with a further assault: "Superficial, impulsive schemes like Sen. Grassley's defense freeze are better suited to bumper stickers than the realities of the dangerous world in which we live." Goldwater had a final put-down: Grassley isn't a member of the Armed Services Committee -- Goldwater is the chairman -- so what can he know? "He doesn't have access to all the information required to discuss the defense budget," said the chairman, who gives access to any general, admiral or military supplier who screams the communists are coming.

Among his New Hartford hogs and while showing a visitor to a barn where the shoats are sleeping and to a pen where a boar is grunting, Grassley prefers to talk about the farm and the beauty of Iowa's springtime rather than the snipes from Goldwater. This is home on the weekend, a moment for renewal of the spirit through contact with the earth.

Iowans, heartened that their farm boy is becoming a national figure, are rallying to Grassley's defense. A letter to The Register last week said that "Goldwater brings out that old argument that Grassley is not a member of the commit- tee so he 'does not have access to all the information.' This 'big-daddy-knows-best' and 'you'd-agree-with-him-if-you-had-the-secret-information-he-has' argument simply does not wash with those of us who were adults during the Vietnam War. Besides, this is a cheap shot which attempts to put Grassley down."

The senator is up right now: in popularity and influence. He is currently the liberals' favorite conservative, a fate he can live with. Goldwater and the Pentagon are dismissing him as a rube, but Grassley's attacks on waste, fraud and excess are seeds sure to grow. He is betting the farm.