For most of the 14 months that Donald Stewart campaigned for the U.S. Senate, scarcely anyone noticed.

The political "experts" and the big money people wrote off his candidacy as a lost cause. The newpapers, more interested in flashier, better financed candidates, ignored Stewart, a Democrat.

"If we went into a town and got two people to sit down and listen to us last fall, we thought it was a successful rally," recalled Carl Ivy, Stewart's campaign manager. "If we had a fund-raiser and five people showed up and gave us $100 each, we were ecstatic."

"People wouldn't pay any attention to us," adds Stewart, a 28-year-old attorney and state senator from Anniston. "They didn't think we had a chance."

But Stewart kept moving across the state, month after month, sounding concerned, reasonable and vigorous. And in the runoff election he scored a stunning upset, defeating Sen. Maryon Allen in her bid to complete her late husband's term

With only taken opposition from Republican George Nicholas, who has never held public office, Stewart until yesterday was virtually assured of election in November. But Nicholas withdrew from the race yesterday and was replaced by former Republican congressman Jim Martin.

Martin, previously a candidate for Alabama's other Senate seat, has a well-financed campaign warchest and is considered a far more formidable opponent. Yet no Republican has ever been elected to statewide office, and Stewart remains the favorite.

His win, by 56 to 42 percent over Allen, was a victory of tenancity, timing and positioning.

Stewart was the sleeper in Alabama's mosts important election year of the century, in which voters turned away from well-known, experienced politicians to fresh, untried faces at almost every opportunity.

He began his campaign as a little known state lawmaker with a moderate to liberal reputation. He was outspoken on a number of issues, including utility company regulations and education. Blasting "the fat cats" and Alabama Power Co., the state's most popular political whipping boy, Stewart courted labor and black support, saying he was running a "people's campaign."

Originally, he was running for the seat held by Sen. John Sparkman, who is retiring after representing Alabama in Congress since in 1930s. George Wallace, who has dominated state politics for a generation, and a former state supreme court justice, Howell Heflin, were the leading candidates in that race. Wallace later dropped out.

When Sen, James B. Allen died, Stewart switched races last June. The immediate reaction was negative. Southern tradition allows an office-holder's widow to complete his term, which Mrs. Allen announced she wanted to do. Her husband had been considered the state's most popular political figure. She was rated a shoo-in, Stewart a hapless opportunist.

But Mrs. Allen never established herself as a credible candidate. She spent most of the time before the Sept. 5 primary in Washington, avoiding press and voters.

When she failed to get the 50 percent vote necessary to avoid a runoff, she announced that she would spend more time in Alabama. But still she avoided confrontation with Stewart or the press.

Stewart capitalized on her ineptitude as a candidate. When she refused to appear on a statewide television debate with him because she would "be washing my clothes home in Gadsden" that night, he blasted her for her "unwillingness to let the people know where she stands, or listen to what they have to say."

When she missed roll calls in order to campaign, be blasted her for being absent from one-fourth of the Senate votes taken since she took office.

Still, Stewart's campaign never generated real excitement. It was waged on a shoestring budget of about $330,000 Stewart borrowed almost $300,000 to keep it afloat.

The, too, the race was overshadowed by bitterly conttested campaigns for the governorship and Sparkman's Senate seat.

When Stewart visited Selma in the closing days before the runoff, there was no crowds to greet him. No hoople no ballons. Just a handful of supporters, and friendly handshakes.

"I like Jim Allen, but I don't think his wife should take his place just because she was married to him," Agnes Bel told him at a drugstore. "I used to work for a bunch of men at an insurance company. Their wives didn't know a thing about the insurance business, and I don't think Mrs. Allen knows much about being a senator,"

Apparently, a lot of other voters thought the same. Stewart is already gearing up for another election in 1980, when Allen's term would have ended.