The tiny plane carrying the tiny Republican candidate for the U.S. Senate was being bounced around by the hit-and-miss air currents rising from the Kansas wheatfield. A bump here, a jolt right after it.

It was a distressing and upsetting journey to a speaking engagement, but in reality, it was a plane trip as shaky and as bumpy as Nancy Landon Kassebaum's Senate candidacy has become.

For as much as she was buffeted by the winds, Kassebaum has been battered by the winds, Kassebaum has been battered by a financial disclosure controversy that has tainted her girl-next-door image and called into question her mettle in a race where she herself acknowledges the main problem is convincing voters she is tough enough to be a senator.

"I have some doubts about it, some qualms whether she could do it," says Harlan Callin, a Hallmark Greeting card employe after hearing Kassebaum's speech at the end of her plane trip the other day. A registered Republican in strongly Republican Johnson County, he should be her bread-and-butter voter, but he adds: "I haven't made up my mind, I was hoping she'd do it for me today.'

The faltering of Kassebaum's campaign has lifted Democrats' hopes of electing their first Democratic U.S. senator from Kansas since the Depression. At the same time, it has been a dismal development for Republicans, who have targeted the race as crucial to holding their numbers in the Senate. The incumbent, Republican James B. Pearson, is retiring this year.

So great are the stakes that Sen. Edward M. Kennedy (D-Mass.) appeared here Monday for Democrat Bill Roy, and President Carter is due here today for Roy. Former president Ford, Sen. Barry Goldwater (R-Ariz.) and House Minority Leader John J. Rhodes (R-Ariz.) all will have appeared for Kassebaum before election day Nov. 7.

Thus has it happened that a campaign that began with promise and ambition has become instead a last minute effort to salvage the image and the candidacy of a woman who carries one of the most revered of Republican names, Landon.

Kassebaum, daughter of 1936 Republican presidential candidate Alf Landon, is now pulling commercials off television and subsituting harder-hitting ones. She is adding staff members and is now attacking the voting record of Roy, who served in the House from 1971 to 1975. She is trying to portray him as a big spender then and fiscal conservative today - all in an effort to regain the initiative with Roy after seeing the polls chart a narrow Kassebaum lead into a 10 percentage; point deficit.

But so volatile does this race remain that when Kennedy vigorously endorsed Roy here this week to the tune of "Happy Days are Here Again," the senator was asked to take his next speech, against right-to-work laws, clear across Missouri to St. Louis, just to keep that criticism away from voter in Kansas, where right to work is certainly law and perhaps religion. (A compromise was struck and Kennedy spoke in Columbia, Mo.)

After all, Bill Roy knows how a good lead at the polls can be lost by election day. In 1974, he led Sen. Bob Dole by about the same margin he leads Kassebaum today. A last minute blitz by Dole, however, scotched Roy and narrowly returned Dole to the Senate.

The scramble this year was set in motion by a routine financial disclosure by Roy. It soon turned into the critical issue in an otherwise issueless race, in which the candidates agree on many topics.

Early in September, Roy, a Topeka physician, disclosed a net worth of $329,000. He provided tax returns showing that he and his wife paid $36,600 in federal and state income taxes and other taxes in 1977 on income of $104,600. He challenged Kassembau to disclose her worth and income and tax returns.

Time passed. Finally Kassebaum disclosed a personal net worth of $2 million, taxable income with her husband for 1977 of $92,289 and said that they paid only $5,075 in federal and state income taxes that year. But she did not release her assets or tax returns, saying that her husband, a wealthy lawyer from whom she is separated, insisted on privacy.

The impact was devastating. First it contrasted sharply with the fresh voice, your-neighbor image that Kassebaum had portrayed so well in her primary election campaign, right down to a TV commercial with her at the kitchen sink and rubber gloves on the faucet.

Roy, for his part, spoke darkly of financial investments that could prove a conflict of interest for Kassebaum in the Senate and referred to how the rich find investments to minimize their tax bills. It has a certain strange twist to it, considering that Roy's party wrote the tax laws.

"My major problem at this point," says the five-foot, two-inch Kassebaum "is just, 'Can a woman serve effectively in the Senate?' There is a question of whether I can go toe-to-toe with them. We did not react fast enough or aggressively enough [to the tax issue] and it reinforced that questioning of whether I was tough enough."

The polls turned on her. In August, after her well-publicized primary election campaign and victory, Kassebaum led Roy 44 percent to 40 percent in a statewide poll by the Central Research Corp. for the Topeka Capital Journal. Then a late September poll by the Kansas City Star gave Roy 47.5 percent, Kassebaum 41.7 percent. Finally, last Sunday, Central Research reported a 10-point Roy lead, 47 to 37 percent.

Perhaps worse, one in four Republican voters surveyed by Central Research favored Roy. "He's probably more qualified to deal with the people he would have to deal with up there [in Washington]," says Republican Callin, the Hallmark employe whose mind is not yet made up.

"When candidates are on the same side of big issues," says Central Research's president, Don Hardesty, "there is a lot of focusing on small issues, and a lot of it [support] is voter comfort - confidence, personality, shared goals." He adds that the financial disclosure issue "redefined" Kassebaum's image, particularly in Wichita, the state's largest city.

Hardesty said, too, that the very nature of Kassebaum's problem, one of image, and the lack of strong party loyalty make it possible for her to reestablish herself and possibly win.

Until her recent effort at being more aggressive, Kassebaum had focused her projected $600,000 campaign on the contention that as the only woman in the Senate she could more effectively work for Kansas. She has avoided running as a women's issues candidate and, indeed has angered feminists by her stand against a time extension for the ratification of the Equal Rights Amendment, which she supports.

She calls inflation the No. 1 issue and says federal spending should be reduced.

Also calling for reduced spending, Roy has argued that Kansas would have more clout in the Senate by electing a Democrat, a member of the controlling party. But he attacks Democratic spending policies and gingerly takes positions designed not to offend the state's conservative voters but still win support from more liberal elements such as labor.

For example, on the controversial labor revision legislation, he favors some parts, opposes other parts, and says he is still undecided on still other problems.

Roy expects to spend $750,000 and has received some $55,000 so far from labor unions.

Kassebaum is now trying to put Roy on the defensive. The labor support, the voting record, his designation by the Connecticut Taxpayers Association as one of the 25 freest spending congressmen while he was in the House, and his calls today is a balanced budget are all themes in Kassebaum's attack.

The Roy campaign has been well organized, professional and tightly run. Kassebaum's is less so, to the point where one person involved goes so far as to call it amateurish. It is a campaign that brought Republican Sen. S. I. Hayakawa of California to a college campus appearance where he urged the election of Kassebaum, "because it's nice to have a lady in the Senate."

A coming joint appearance by Goldwater, who unsuccessfully ran for president in 1964, and Alf Landon is jokingly referred to by Democrats as an appearance by two men who together lost 92 of 98 states in their presidential bids.

Hardesty, the pollster, speculates on a close race, one in which perhaps the deciding issue has not yet emerged. That is, unless Kassebaum cannot shake the damage of the financial disclosure issue.

She properly notes, as she did the other day in Manhattan, Kan., as she cheered on her precinct workers, that while campaign workers win an election, "the candidate can lose it."