For 16 years as a congressman and 24 years as a senator, Nebraska's Carl T. Curtis has been a featured speaker at the annual Founder's Day GOP rally here. Saturday's was his last as an incumbent, for Curtis has announced his retirement.

"If I live until Jan. 3 when the term ends," he told a visitor, "I will have served in Congress two months longer than George Norris."

One had the feeling that for the conservative Curtis, eclipsing the record of Nebraska's great progressive senator would seem a final victory.

But for others in the Founder's Day crowd, next Jan. 3 promises to set another record, not much to their liking. Unless a remarkable upset occurs in the November voting, that day will also be the first time in Nebraska's 110 years of statehood that his bastion of Republicanism, formed in the passions of the Civil War, will have two Democrats in the U.S. Senate.

Gov. J. James Exon, a Democrat is strongly favored to defeat Curtis' administrative assistant, Don Shasteen, and join Edward Zorinsky, who in 1976 was elected Nebraska's first Democratic senator in 40 years.

Another political eclipse could occur even sooner. Richard White, the state Democratic chairman, sees a chance that the registration figures for the May 9 primary may show, for the first time, more Democrats than Republicans. The Republicans' registration margin has melted from 47,000 in 1968 to 30,000 in 1972 to 17,000 in 1976, and could disappear entirely this year.

Optismistic Democrats think Exon's personal popularity will help Lt. Gov. Gerald T. Whelan retain control of the governor's office. Freshman Rep. John J. Cavanaugh, the first Democrat elected to the House since 1964, seems to have entrenched himself in his Omaha seat.Former Democratic Party chairman Hess Dyas is regarded as a strong favorite to win the Lincoln seat of Rep. Charles Thone, who is quitting to run for the GOP nomination for governor. If that parlay worked, it would leave Rep. Virginia Smith of the western rural 3d District as the only Republican in top federal or state office.

But the Republicans who gathered here on Saturday do not accept that as inevitable. While few would endorse Curtis' prediction of "pretty much a Republican sweep," the optimism expressed by speaker after speaker did not seem entirely synthetic.

Exon's departure gives the Republicans a good shot at the key job - the governorship - which they have held for only four of the last 20 years.

President Carter was trounced here in 1976 and even leading Democrats concede that his administration's farm and fiscal policies have done little to help the Democratic image.

Except for the Senate race - where Exon's popularity scared off all potential GOP challengers except Shasteen the Republicans have an abundance of candidates. The primary contestants speaking at Founder's Day included young legislators, mayors, farmers, teachers, several women and even a couple of blacks.

Thus, Nebraska in 1978 is a microcosm of the national Republican picture, reflecting both the damage the party has suffered in areas of traditional strength and the possibility, but not the assurance of a turnaround in 1978.

Ask a dozen politicians of both parties why the Republican position has deteriorated so badly in Nebraska in the past decade or so and there is broad agreement on the answers. A combination of political complacency, ideological and personal factionalism and organizational rigidity on the GOP side enabled the hard-working Democrats to alter the basic power balance in the state.

"We've just let our organization slip," says Thone, who was party chairman back in the late 1950s when the decline began.

The pace ofthe falloff has been drastic. In the last three gubernatorial elections - 1966, 1970 and 1974 - the Republicans' share of the vote declined from 61.5 percent to 44.8 to 27.4.

In 1964 and 1965, Republican Sens. Roman L. Hruska and Curtis respectively were reelected with slightly more than 61 percent of the vote. In 1970 and 1972, the figure was barely 53 percent. In 1976, when Hruska retired, his would-be successor, Rep. Johy Y. McCollister (R-Neb.), could muster only 47.5 percent of the vote against Zorinksy. Few in either party believe that Shasteen will come as close to Exon in this year's battle for the Curtis seat.

As these figures indicate, the decline has been pervasive. There has always been a healthy Democratic vote in Omaha, but in recent years the party has built its strength across the state, drawing young people who had little loyalty to, or contact with, the aging Republican incumbents.

Democratic chairman White says: "The Republicans had been in power for a long time. It was very difficult for young people to advance in their structure, so they looked to us. From the 1950s on, whe have had an early presidential primary. John Kennedy, Bob Kennedy, Gene McCarthy and George McGovern all attracted people into the party byp running here and gave them an interest in campaigns.

"Since the early 1960s, and particularly since Jim Exon's been governor, we'hve had a really active state organization, with support from the governor's office and consistent leadership."

As Republicans looked at the picture, 1970 was a key year in the downfall of their party. In that year, Gov. Norbert Tiemann, a progressive Republican who had introduced a state sales tax and income tax in his first term, was challenged for renomination by conservative state Sen. Clifford Batchelder. Tiemann won narrowly, but, as he said the other day, "After the primary, that element never supported me," and he lost the general election to Exon.

The Tieman-Batchelder split, symbolic of the progressive-conservative division inside the GOP, has not healed. Anne Batchelder, wife of Tiemann's challenger, is party chairman now, and Tiermann has told Nebraska newspapers that under her "the party is in complete shambles. They are not going to have any winners until they get over the inflexible ideas that if the candidate doesn't walk, talk, think and say exactly the way they want him to, they won't back him."

Although some conservative Republicans echo Tiemann's view privately, Batchelder dismisses it as the expression of "a sore loser" and says it was Tiemann's spending policies that enabled Exon to gain office on a platform appealing to Nebraska's traditional conservatism.

In 1976, the GOP was divided again by the Ford-Reagan contest, which Reagan won by a narrow margin. Worse, when the dominant GOP faction backed McCollister as Hruska's Senate successor and actively discouraged Zorinsky from challenging him in the primary, the marverick mayor of Omaha, a lifelong Republican, bolted to the Democrats and beat McCollister in November.

"The Republican leadership," Zorinsky said, "had got to the point they thought they could appoint a senator, rather than have to elect him."

The real test of whether Republicans can begin to decover from these reverses is the governor's race. Nationally, the Republicans control only 12 governorships, and GOP officials in Washington warn that without a dramatic improvement at that level, the party's long-term future would be in jeopardy.

The Nebraska governorship is certainly winnable. Leading Democrats concede that Whelan, the lieutenant governor and party nominee, has been overshadowed by Exon's personality and has yet to establish an independent following. The question is how strowng and unified the Republican challenge will be.

Thone, a longtime Hruska ally and farm spokesman, is regarded as the front-runner in the GOP primary and is the favorite of the party establishment. Thone says he would use the governorship to revive the weakened GOP organization. But some Republican leaders doubt privately that his low-key, traditional campaigning can capture younger voters.

Curtis, who has always had his personal differences with fellow-consevative Hruska, is backing his own candidate for governor, Vance Rogers, a minister and former president of Nebraska Wesleyan University. But Roger's candidacy is thought to be lagging.

There are two "outsiders" in the race who are given a chance of winning and upsetting the Republican apple cart.

One is Stan Juelfs, a wealthy farmer and oilman who is running an anti-politics, anti-big government campaign with $300,000 of his own.

The other is Bob Phares, a former mayor of North Platte who, at 38, is the youngest man in the field. He has put together an organization of mayors, Junior Chamber of Commerce members and other young people, and is regarded by some key Democrats as potentially the strongest challenger to Whelan.

Phares says, "Republicans have to start saying what we're for - not just what we're against." In a surprise coup, he gained the endorsement last week of Milon Bish, the 1976 Reagan chairman, dismaying Julfs, who had been a major Reagan contributor.

Whether any of the four Republicans can unite the party in November is a matter of great private concern among GOP officials. If no, next Founder's Day will be a pretty bleak occasion.