"Do you have to cuss and chew tobacco to be heard in this town?" a congressional aide asked last week, reflecting on the quiet style that has always set his boss, Sen. Nancy Landon Kassebaum (R-Kan.), apart from her colleagues.

But the question was rhetorical: As the sponsor of a bill to impose limited sanctions against South Africa, Kassebaum has lately become an important bellwether of Republican feeling in the Senate -- and has emphasized her emergence as a voice to be heard on foreign affairs.

Heard but not listened to at the White House. On the eve of President Reagan's speech on South Africa last Tuesday, Kassebaum, who chairs the Senate subcommittee on African affairs, and two other senators met with Reagan and warned him against rejecting sanctions against South Africa. They told him that if he did not put pressure on Pretoria, the Republican-controlled Senate would follow the House in enacting sanctions. The president's speech didn't reflect the advice, and Kassebaum said she was "deeply disappointed."

The importance of Kassebaum's visit -- like that of her fellow Senate Republicans Richard G. Lugar (Ind.) and Majority Leader Robert J. Dole (Kan.) -- is that her recent calls to action on South Africa represent a reversal of her long-held position on that issue.

In January 1985, Kassebaum said, "I don't believe economic sanctions are the answer. I don't see it forcing South Africa in any different direction. That's what I see reinforcing their bunker mentality, and may well force severe economic disruption."

Now, angered by the state of emergency declared June 12 by the Pretoria government and exasperated with the failure of the U.S. "constructive engagement" policy to bring about reforms, she has endorsed limited sanctions such as denying landing rights to South African planes and barring new investment in South Africa. Some version of these proposals is considered likely to pass the Senate. And if these were to fail to achieve results, Kassebaum does not rule out a total embargo.

She is also prepared to consider other forms of action. "As The Economist points out this week, if we're really serious about doing something dramatic, we would sell the gold reserves that each of our countries hold," she said. "And if we start to move those out on the market, the price of gold would certainly come tumbling down.

"I don't endorse that dramatic approach at this point because I think the repercussions would be enormous. I am still torn and still would argue that it won't help anyone to have South Africa reduced to economic chaos." But she added: "You can't totally rule it out."

For a woman who has a reputation for shunning publicity, Kassebaum is in the center of controversy. For one whose instincts are conservative, especially in foreign and fiscal policy, she now speaks eloquently against the administration position.

Colleagues and others who deal with her have a simple explanation: Kassebaum, they say, has a rare independence. She has demonstrated it on a series of issues from abortion, on which she takes a pro-choice position, to the nomination of Daniel A. Manion as a federal appeals court judge, which she opposed.

This has caused her to run afoul of some of her constituents, who feel she is too liberal, but she won reelection in 1984 with 76 percent of the vote. Her aides and others who know her well said that she has managed to retain her popularity because she is seen as being sincere in what one aide described as her Hamlet-like struggle with issues.

Kassebaum's life in politics is partly the legacy of her father. Alf Landon was governor of Kansas from 1933 to 1937 and ran for president in 1936, but was able to take only two states. As a child, she would listen by a ventilator in her bedroom to dinner conversations, where the topic was always politics.

She discusses issues with her father, now 98, but said of him, "Although he denies this vehemently now, he was not one who ever thought there would be a woman in the U.S. Senate, much less his daughter. He comes from an era where it is just hard for him to adjust to talking politics to me." She laughed, "It doesn't bother me, 'cause I understand him."

Kassebaum, who turns 54 today , holds a master's degree in diplomatic history from the University of Michigan. She then settled down to domestic life and had four children.

She was divorced in 1979. Her entry to the Senate that year owed much to her father's name; her qualifications -- one year as an aide to former senator James Pearson (R-Kansas) and service on a Kansas school board -- were not rated high by political observers. But she has since become popular in her own right, as her easy reelection attests.

Her constituents like her for, among other things, her love of Kansas, where she returns every other weekend to her parents' home, or to her farm, or another part of the state. One aide said: "Even if she had a chance of a trip that would take her to Italy, Greece, Hong Kong and Taihiti, she would prefer to go to Topeka.

"She is atypical of Washington . . . She has a modest house in a nice neighborhood, has no maid and drives a modest car. She does not go to power-brokering parties. She is not into Washington." He added, "If the job is not the beginning and end of your life, you can afford to be courageous."

One of only two women in the Senate, Kassebaum dislikes tokenism. When asked to attend opening ceremonies at the Republican National Convention in Dallas in 1984, she is reported to have said that she did not want to attend merely as window dressing, "a bauble on a tree."

Even today, Kassebaum's stance on South Africa cannot be called radical. The sanctions she proposes are far more moderate than those passed by the House last month. And she thought Secretary of State George P. Shultz's statement on South Africa last week before the Senate Foreign Relations Commitee -- which most observers thought was only marginally tougher than the president's speech -- contained good ideas and that the president's speech would have been better received if he had had a similar emphasis. Shultz's message was "unfortunately a bit tarnished because of the president's speech," she said.

"Obviously one of the keys, I believe, to negotiations is the release of Nelson Mandela because he is the one leader -- I suppose just by the fact that he has been in prison for so long -- who has risen to heroic proportions and can pull a lot of factions together."

But Kassebaum is above all else a realist. She cautions against thinking that sanctions will bring a turnaround in the South African government's policy.

"We would be fooling ourselves if we really thought it was going to cause South Africa to cry uncle," she said. "But I think at this point it is necessary for us to to try a different mix of pressures, to really clearly signal whose side we're on."