Once as secure on the southern Arizona landscape as the giant cactus, Rep. Morris K, Udall (D-Ariz.) has watched his formidable election margins wilt slowly over the last decade in the heat of Sun Belt conservatism.

Now the cactus is threatened and so is Udall, the tall, lanky, 58-year-old liberal whose national ambitions and prominence have strained his foothold in the shifting sands of his desert constituency.

Republicans, sensing their time has come, are giving him the toughest race of his 20-year House career in the person of Richard H. Huff, a wealthy conservative Tucson realtor who is capitalizing relentlessly on the liabilities of incumbency, power, prominence and liberalism in 1980.

"I've about decided from his ads that if you have falling hair, dandruff, loose dentures, low-back pain or a lousy sex life, Mo Udall caused it, he meant to cause it, and he enjoys seeing you suffer," Udall complained to a radio interviewer the other day.

Maybe not falling hair and dandruff, but Huff is trying to make Udall, the leading liberal challenger for the Democratic presidential nomination in 1976 and the environment-minded chairman of the House Interior Committee, the lightning rod for voter dissatisfaction over just about everything that is coming out of Washington these days.

For instance, just as Udall was sounding off to the radio interviewer, Huff was going before television cameras at a Sears store with a package of meat, a pair of overalls and a can of paint, telling people how much they cost when Udall went to Congress in 1961 compared with their price today.

And the airwaves have been filled with radio ads that run this way:

First voice: "Mo wouldn't want to shortchange the defense budget. That's not the way we think in Arizona."

Second Voice: "This Richard Huff newsletter says Mo voted to kill the B1 bomber. A lot of people are wondering if we still have the defense strength we used to."

First Voice: "But Mo knows that. After all, he's been our congressman for 20 years."

Second Voice: "That's the point Huff's making. Huff says Mo's forgotten what it's like to live in Arizona. That's why Mo voted with his East Coast pals for the biggest federal budget in history. And Huff cites the Congressional Record!"

First Voice: "For spending and against defense? Hey, let's call Mo."

Second Voice: "I'm calling his Tucson office again. It's 792-6404. . . ."

As Udall and his supporters cry foul at such tactics, Huff responds, "If he isn't responsible, who does Mo think we should hold responsible?"

While Udall appears to be getting the benefit of the doubt so far, the race could be close -- or an upset -- nonetheless.Udall leads by 15 percentage points in his own polls and by 25 points in an Arizona Republic newspaper survey last week. But perennial difficulties in turning out Hispanics and other Democratic constituencies -- complicated by President Carter's lack of popularity and effort in Arizona -- could be such a problem that a shift of 5 points would spell real trouble for Udall, according to campaign strategists.

Udall is not alone among Democratic congressional leaders in feeling a backlash against incumbency and power. He is joined, in varying degrees of jeopardy, by such House leaders as Majority Leader Jim Wright (D-Tex.), Majority Whip John Brademas (D-Ind.), Democratic Caucus Chairman Thomas Foley (Wash.) and Ways and Means Committee Chairman Al Ullman (D-Ore.) -- as well as by a half dozen or more prominent Senate Democrats.

Udall's position may be more tenuous than the other's however.

His bid to rally the Democratic left against Jimmy Carter in the primaries four years ago, reinforced by his support of Sen. Edward M. Kennedy (D-Mass.) and his Democratic National Convention keynote speech this year, led some home folks to believe that "this good old guy has turned into a foam-at-the-mouth liberal," as Udall ruefully put it recently. Udall contends that he's largely overcome this problem, but others aren't so sure.

At the same time, the massive immigration of relatively affluent retirees and middle-income employes of the area's new high-technology industries has gradually eroded Udall's old Democratic base and infused the district with people who know Udall only as a national figure, not the "good old guy." Tucson, the heart of Udall's 2nd Congressional District, has grown 10-fold since his first election.

For these and other reasons, Udall's victory totals have plummeted from 70 percent in 1968 to about 53 percent in 1978, with an average decline of more than 3 percentage points per election, or enough of a drop to beat him this time.

And he's never before had an opponent quite like Huff, a fervent conservative, born-again Christian and aggressive campaigner who's hired a hard-hitting media consultant and inspired the support of Moral Majority and other similar groups of the New Right.

Huff has made trips into the Appalachian coal fields to raise money against Udall, who has infuriated the coal industry with strip mine and other legislation. Alaskans like former Interior secretary Wally Hickel, angry over his Alaska lands bill, are helping Huff's campaign, as are oil and gas interests. Huff says he's been told that he has a longer list of business political action committee supporters than any other Republican this year.

A 54-year-old native of West Virginia who came to Arizona in 1962, Huff sold his real estate business at age 50 and decided to devote his new leisure time to politics.

Udall and Huff have worked up such a dislike for each other that they campaign like two tarantulas in a bottle. Udall, who comes across like a gentle giant in Washington, accuses his rival of "lies and outright distortions . . . an unscrupulous campaign of slogans, fear and not much else." Although he keeps reminding himself to get off the defensive, his campaign has a reactive, slash-back quality that feeds the notion that he's vulnerable.

Huff accuses Udall of voting against the interests of everyone from unborn children to senior citizens, describing him as a big-spending, anti-defense, pro-preservationist liberal who has an "uncanny ability to make people [in Arizona] think he agrees with them when he doesn't."

Huff is aided by the fact that he is running on the same ticket with Republican presidential nominee Ronald Reagan, a prohibitive favorite over Carter in Arizona, and Sen. Barry Goldwater, who has long ranked close to the Grand Canyon as Arizona's most treasured resource.

The Carter forces have so forgotten Arizona that when Udall, anxious not to appear as though he were slighting his old rival, demanded a Carter poster for his Tucson office, aides had to borrow one of the three that they managed to find in the state.

But Goldwater is another matter. At age 71, and plagued by a hip ailment and a poor attendance record in the Senate, he's having reelection problems of his own. Perhaps more important, he doesn't bother to hide the old family ties that bind him and Udall.

When Goldwater was asked for his views on the Udall-Huff race the other day, he told the story about how his grandfather helped get Udall's grandfather, a Mormon arrested for polygamy, out of jail, and added that "Mo is a very powerful man in the United States House -- let the people be the judge." However, he later affirmed his support for Huff, saying it's "long past time that Mo came home" -- which is roughly what Udall is saying about Goldwater.