A beaming, denim-clad figure -- his arm oustretched in a big Western-style salute to the crowd -- waves out at Idaho from hundreds of posters this summer.

No, the rodeo's not in town. It's just Senate Foreign Relations Committee Chairman Frank Church back home on the range, hustling for votes.

For four senatorial terms spanning nearly a quarter of a century, the 55-year old Church has climbed the ladder of influence in Washington, suave in manner and global in interests -- all the while keeping his feet firmly planted in the grass roots of wheat-and-potatoes, rivers-and-dams and guns-and-prayer politics of this sparsely populated intermountain state.

A Democrat among Republicans, a liberal among conservatives, he has bridged the chasm between Idaho and Washington in a manner that admirers call brilliant -- and critics call shameless.

One piece of campaign literature wraps it up pretty well. "Frank Church Says No to Gun Control . . . Idaho Never Had a Better Friend," reads the flier, emblazoned with a picture of a parka-clad hunter packing a rifle and scanning the horizon for prey -- a Frank Church whom few Washingtonians would recognize.

Twin Falls Mayor Hank G. Woodall, who says he usually votes Republician, put it another way as Church made a handshaking sweep of the lunch crowd at a downtown restaurant the other day -- recognized and greeted by name by many townsfolk.

"In Idaho, there's Republicans, there's Democrats and there's Frank Church," said Woodall. "A senator from Alabama could do the same things and be a bad guy. But Church, he's a local boy . . . He does well by Idaho."

Whether by conviction or constituent pressure, he strays from the liberal often bigger than life in his Western mountains: to allow prayers in school, to prevent gun control, for water projects that are pure gold to the West and pure pork to the East, against federally funded abortions.

Washington looks on him as the defender of SALT II; Idaho sees him as the hero of Hells Canyon, Sawtooth and Gospel-Hump.

This good neighbor, good-provider image may well pull Church through what is expected to be a perilous year for a liberal or even a moderate -- the latter designation being favored by Church, who glides quitely toward the center at election time.

But right now both Church and his Republican challenger, four-term Rep. Steven D. Symms, agree that there may be no more than even-money odds on the outcome -- a race so close and tense that it has already produced a bumper crop of invective.

At 42, with boyish good looks to go along with his background as a third-generation fruit grower from southwestern Idaho, Symms was clearly on the offensive until last month when Idaho newspapers began publishing stories about Symms' investments in the silver market.

They reported that he made a profit of nearly $10,000 in silver futures trading during the same period that Nelson Bunker Hunt, who serves on Symms' national finance committee, was speculating massively in silver. Symms, the stories noted, serves on House committees that deal with minerals and commodities trading.

The newspapers also reported that Symms voted against legislation extending the life of the Commodities Futures Trading Commission, which oversees silver speculation, and backed legislation that would have guaranteed a fixed share of the sugar market for producers, including a Hunt-controlled company.

Symms cried foul, denying that he violated any House ethics rules. Shortly after the stories broke in June, he told the Idaho Statesman newspaper, "The public could care less about this stuff. I could care less . . .." But in an interview this week, he was more circumspect. "Like being tarred and feathered," he said, "but for the honor I'd rather go without it."

While it's too early to assess the impact of the stories in this silver-producing state, Church supporters are having a field day. A pro-Church mayor ran up and down the street during a parade in his home town recently, shouting, "The only silver Frank Church has is in his hair."

In his attack on Church, Symms has an ideologically pure message: Church is too liberal for Idaho and uses his influence on behalf of "whiteflag diplomacy" abroad and a "welfare state" at home.

Fighting words for Idaho, perhaps, but nothing compared to the 15-month drumbeat of radio and television commercials and mailouts from New Right groups that portray Church as a wrecker of the Central Intelligence Agency, saboteur of the military and all-around left-winger.

Obviously nettled, Church has lashed back at what he calls the New Right's "big lie" campaign against him and suggested that Symms is a congressional lightweight so beholden to oil industry campaign contributions that he would be a "senator from Exxon" rather than Idaho. At one point, Church got so carried away that he wrapped himself in red, white, and blue bunting to ridicule Symms' flag waving, prompting local American Legion officials to demand an exlanation.

But this week Church, armed with the results of a recent campaign poll, said he believes he has broken Symms' earlier momentum and now has a "small lead." In popularity, he said, he was running only slightly behind Ronald Reagan, who will be nominated next week by the Republicans for president. That is the good news. The bad news is that President Carter, who will be Church's Democratic ticketmate, got only 12 percent of the vote in the poll.

"I'm running against a Republican tidal wave," Church complained.

What is most difficult to assess is the strictly negative, anti-Church campaign orchestrated by the Virginia-based National Conservative Political Action Committee (NCPAC), working in Idaho through a home-grown affiliate known as Anyone But Church (ABC) and a bevy of antiabortion, antiunion and other single-issue groups. A pro-Arab group has even weighed into the fray, accusing Church of submitting to "Zionist pressures."

NCPAC acknowledges spending $140,000 against Church, and Church claims the New Right spending is more like $250,000.

At ABC's Boise headquarters, Jake Hansen, its executive director, proudly shows two piles of clippings from letters-to-the-editor columns for one week: four for Church, 37 against him. "We're having an impact, clearly," said Hansen.

Symms says he has his doubts, but Church contends that ABC's campaign has had a "corrosive effect," attributing it to an "exploitation of frustrations, a feeding on people's suspicions, that creates a far more vicious attitude than would be natural for people of this state."

ABC had to backtract on its charges in at least two cases, including one television spot taped in front of an empty IBCM silo, implying that Church was responsible for the vacuum, although the silo turned out to be part of an abandoned Titan system network that has been replaced by Minuteman missiles.

Most voters interviewed on a two-day swing through Idaho shrugged off the ABC campaign. "People here are too sophisticated for that kind of stuff," said Twin Falls Mayor Woodall.

More so than most of his five liberal colleagues on the New Right's hit list, Church has maintained home state roots and branches so thick that previous challengers have gotten lost in the goodie-bedecked forest that Church has nurtured over the years.

A leaflet entitled "Getting the Job Done for Idaho" (with a picture of Church in shirtsleeves, on the telephone) mentions, among other things, keeping Idaho's water in Idaho, preserving Amtrak service in Idaho, passing a rangeland rehabilitation bill and winning money for the American Falls Dam, for experimental bulb turbines at Idaho Falls and for geothermal projects at Raft River and Boise -- to say nothing of parks and wilderness areas all over the Idaho Rockies.

He visits Idaho regularly, even when not running for reelection, and holds Christmas parties every year for senior citizens. "His constituent services are excellent," concedes a Symms aide.

Church created a stir last year in Idaho as well as Washington when, speaking from Boise, he disclosed the presence of a Soviet combat brigade in Cuba shortly after being informed by Washington -- an act widely interpreted as an election warmup gesture toward his hawkish home state.

This, coupled with other votes, including votes against two liberal federal judgeship nominees, provokes charges of waffling from his opposition. But Church calls the criticism about his Soviet troops disclosure a "bum rap . . . mainly from the Eastern press" and says, "The things that matter are the things I've stood for consistently because I believe in them." m

He cites his support for the Panama Canal treaties, which he vigorously defends in Idaho, even though the treaties appear to be political poison in the state. And, at a meeting with farmers in Burley the other night, he got strong applause for a critique of military priorities that including calling the B1 bomber a "turkey."

Still, being chairman of the Foreign Relations Committee, for all its prestige and perquisites, can be a mixed blessing. He gave up a trip to Japan with Carter and other national leaders in part to keep campaign commitments here. But when he got to a local newspaper office, the reporter asked about Afghanistan, Moscow, Europe -- just about everything but Idaho. a

Church had even thrown out the disclosure of a $1.5 million neighborhood improvement grant to the town as bait. No nibbles. Back in the car, Church sighed and said wistfully, "Sometimes I wish they would ask about Idaho."