Sen. Howard W. Cannon, who at 70 tells everyone he is "proud to be a senior citizen," was working the lunch crowd at the senior citizens' service center on 9th and Sutro.

He led a chorus of "Happy Birthday" for all those present and adding a year to their impressive totals. His staff placed little cupcakes decorated with his name on the long, yellow-covered tables.

After 24 years in the Senate, Nevada's senior politician was shaking hands with the sincerity of a man in the political fight of his life, and one of the most divisive and significant campaigns ever in this state of boom and bust.

But he was not getting through to Fred Pattee, 88, a retired decorator, who moved to Reno from South Dakota with his wife, Hilda, 80, because he liked to gamble. With a look of mild disdain inherited from his Sioux ancestors, Pattee quietly dismissed Cannon's energetic appeal to his generation.

"Youth must have its way," he said. Pattee began to praise Rep. James D. Santini, 45, Cannon's opponent in the Sept. 14 primary and the only other Democrat in the state who can approach the senator's popularity.

"I like Jim Santini," Pattee said. "He's very straightforward. He does what he says he's going to." Then, with a smile at his wife, he added, "And he sent us a letter on our 60th wedding anniversary."

Rarely have the relative virtues of youth and age been so hotly debated in a political campaign, rarely has a state party been forced to choose between its most proven vote getters, but Cannon and Santini have leapt onto the issue of seniority and loyalty like tourists from Hong Kong approaching the Las Vegas airport slot machines. They want to pull all the levers and see who wins.

According to Santini's staff, a recent survey by Republican pollster Richard Wirthlin shows the younger candidate with a lead of 6 percentage points, a margin that Santini and Wirthlin agree makes the race too close to call.

Cannon cites other polls showing him slightly ahead, but for the moment the momentum seems to be with Santini, whose energetic, almost manic campaign and speaking style contrasts so greatly to Cannon's slow, courtly manner.

Cannon also can do little about the noticeable bulges in a once-trim body of the pilot hero of World War II who evaded capture for 42 days after being shot down behind German lines.

Like other aging senators before him, Cannon has called in old political debts, has relied on thousands of old friends and has reminded his audiences continually of Nevada's loss of clout if the voters turn out the Senate's seventh-ranked member in seniority.

When a caller asked a woman in his Washington office when Cannon was next scheduled to be in Nevada, the woman sounded amazed at the question.

"He's always there now," she said. "He only comes back here if something big breaks."

His white Ford van labeled "Mobile Office of U.S. Senator Howard W. Cannon" sat under the shade of a tree in the oasis-like town of Winnemucca. It was an old stop for pioneers who had to ford the Humboldt River, transformed now into a popular, motel-rich overnight resting place for Interstate 80 motorists making the long drive between San Francisco and Salt Lake City.

Beside her small swimming pool, entertaining 40 or 50 local people come to meet the senator, was Cannon's cousin-in-law, Betty Cannon, 44. Her husband, Barney, a veterinarian, and the senator had the same grandfather but different grandmothers, a remembrance of the old days of official Latter Day Saints polygamy, she explained with a grin.

She had voted in the past for Santini, since 1974 the state's only member of the House of Representatives, but she was not happy with his attempt to move up to the Senate.

"I think he's cheating Nevada," she said, "not only out of Sen. Cannon's seniority but his own seniority in the House. I think it's a poor decision." She paused. "I hope it's a poor decision."

Nevada's population exploded by 60 percent in the 1970s, the highest growth rate in the country, and is now more than 800,000. This year for the first time it will elect two representatives.

But Santini, fired by what he called "a great need for change" in the state's Senate representation, rejected an easy race for one of those seats and left many old supporters with a dilemma.

His Reno office manager, Lynn Atcheson, said, "It has been diffi- cult for some people. We have some who were dear friends, supporters in past campaigns, who said, 'Not this time.'"

The lone comfort for Democrats is that either Cannon or Santini is expected to beat the winner on Nov. 2 from a relatively weak field of candidates in the Republican primary.

Cannon has built a reputation for admirable shrewdness, developing an expertise and influence in defense and transportation policy that fit a state heavily dependent on military installations and tourism.

But at the Winnemucca party Cannon cloaked his cool, urban intellect in a folksy manner and wore brown slacks, checked shirt, a huge western belt buckle and a string tie with a clasp resembling a map of Nevada. His brown hair appeared to be graying at the roots.

By contrast, the next day in Tonopah, a mining town cradled on a shelf of the desert-locked San Antonio Mountains, Santini presented his own, much different image. He charged late into a fund-raiser in the old hall the town now calls its "convention center." He wore conventional slacks and coat, but without a tie.

The Santini posters plastered about the town, unlike Cannon posters, showed the candidate's face, but it was a stern, professorial image distinguished by severe eyeglasses and prematurely white hair. At the fund-raiser, Santini was more himself, red-faced, hair windblown and uncombed, bemused and sometimes seemingly bewildered, like a lecturer who had lost his notes.

But, as he has in most of the campaign, Santini took the offensive on the issues. Among the 200 people who had paid their $15 for a lasagna dinner, the candidate and his aides berated Cannon for failing to fight the since-discarded plan to tear up much of the desert for a hide-and-seek MX missile system.

They mentioned Cannon's last-minute vote for the Panama Canal treaties, still highly unpopular in a state where Democrats are almost as conservative as Republicans. They referred vaguely to the "integrity" issue, a reminder to voters that Cannon was investigated, though completely cleared, in 1980 of suggestions that he had been bribed by Teamsters union officials.

In Winnemucca, and at the senior citizens center in Reno, Cannon and his aides repeated how much he had done in Congress for their areas, reminding the citizens of Winnemucca in a special fact sheet that he had frustrated attempts to end Amtrak service to the community.

In a campaign with a great deal of fund-raising for such a small state -- $1.2 million collected by Santini and $850,000 by Cannon -- Cannon's aides made much of a $130,000 separate campaign by the National Conservative Political Action Committee to defeat Cannon as one of their target liberals. A Cannon handout said the committee was "devoid of any Nevada contributors."

"I have most of the party regulars with me," Cannon said in Winnemucca, "because they don't like to see this kind of race."

In Tonopah, as the Santini fund-raiser diners tucked away the last of their lasagna, the younger candidate rose to ring the same loud chimes he has rung in every Nevada mining town.

Foreign producers were taking business from U.S. miners, Nevada's second leading industry after tourism, and the Senate needed someone to help save them, he said.

"The GS17 that rules the world in Washington, D.C., says this is a rational allocation of international resources," Santini said, his voice high and excited, feeling the crowd with him.

"Horse manure!" one listener shouted.

"That's exactly what it is!" Santini said. "Maybe we'll make that a slogan of this campaign."