ETROPOLE, BULGARIA -- "Is it true that Communists have ways of detecting fingerprints on ballots?" asked a nervous nurse in this copper-mining town.

Tzvena Kolarova, who had popped into the local headquarters of the anti-Communist opposition, said she will vote against the party that has ruled this country since 1945. Yet, like many Bulgarians who will cast ballots today, she was afraid that the authorities had devious means to find out how she voted.

In the final days before Bulgaria's parliamentary election, which marks the finale in Eastern Europe's historic season of the ballot, reform Communists here have felt compelled to say that they are not plotting anything sneaky.

A statement from the dreaded Ministry of Interior said this week that, no, the authorities were not using invisible ink to render ballots invalid, nor were they using a secret dye that would make color-coded ballots change color, nor would the national electricity supply be cut off when ballots are counted, nor would viruses be released to infect vote-tabulating computers.

Sunday's election, the first free vote in this Balkan country in half a century, is shaping up as the region's first real horse race between reform Communists and an anti-Communist opposition.

In East Germany, Hungary, Yugoslavia, Poland and Czechoslovakia, discredited Communists were more or less dead in the starting gate. In Romania, there was no real opposition to the reform Communists who toppled dictator Nicolae Ceausescu.

But in Bulgaria, there are credible reform Communist candidates who have had some success in distancing themselves from the past. The opposition Union of Democratic Forces has managed to reach out across the country with a campaign that proclaims "45 years are enough." The opposition this week organized the largest political rally in Bulgarian history. About a half million people shouted in the streets of the capital, Sofia, that "communism is over."

Opinion polls, which are considered unreliable, give the reform Communists an edge to pick up the majority of the 400 seats in the Grand National Assembly, but insiders in both parties say they are not sure what is going to happen.

The wild card in Bulgarian politics is fear.

In a corner of Europe that has had little experience with governments that are anything other than brutal, intolerant and authoritarian, fear seems to be a national birthright. A Bulgarian proverb, which emerged out of five centuries of Turkish occupation, advises: "If you bend your head, you will not lose it."

More recently, Bulgarians were terrorized by a network of party-run concentration and labor camps that killed at least 18,000 people while operating from 1944 to 1962. Fear continues to percolate up through the civil rights that were won only last November, when Todor Zhivkov, then the longest-serving Communist leader in the world, was toppled by the reformers who are seeking voter approval today. Kolarova, the small-town nurse who frets about fingerprints in the voting booth, says she has a hard time believing that the ruling party, if it loses, will step aside.

Reports that the reform Communists are playing on fear have provoked sharp criticism from human rights groups and the U.S. government in the past week.

Complaining that "mounting incidents of harassment and intimidation are an increasing concern," the State Department has demanded that government leaders go on television and assure the Bulgarian electorate that the vote indeed will be secret.

The Bulgarian government insists it has done everything possible to ensure fair elections. Indeed, for a government that once had one of the most unsavory, anti-Western reputations in the Eastern Bloc, Bulgaria has tolerated a surprising level of American involvement in the election.

The National Endowment for Democracy, an agency funded by Congress, has spent about $1.7 million here to supply equipment, newsprint and expertise for the campaign. Most of the support has gone to assist the opposition and election monitoring.

"We are trying to understand how democracy works, and American support for democracy here is very desirable," said a senior adviser to Bulgarian President Petar Mladenov, who led last year's palace coup against Zhivkov.

The adviser, who did not want to be quoted by name, said that "with the whole world watching Bulgaria, we are not going to get red in the face about what the Americans say. But of course there is a limit. . . . For them to tell us to go on television and use the word 'secret' is more than advice, it is dictation."

Reform Communists argue -- with some support from Western human rights groups -- that fear in Bulgaria cuts both ways. Many party members, of whom there are more than 1 million in this country of 9 million people, say they are afraid of revenge.

"My mother and father were killed by fascists in 1945 {before the Communist Party takeover} and I am afraid that if the Union of Democratic Forces wins, my personal tragedy will be repeated," said Giorgieva Kalina Ivanova, an engineer and party member.

Helsinki Watch, the human rights monitoring group, said in a report last week that "tensions are exacerbated by numerous instances of physical and psychological violence, sometimes perpetrated by local government officials and other times by party activists. Members of the Bulgarian Socialist Party {which is what the reform Communists renamed themselves this year} are not immune from attack."

The worst offender, according to human rights groups and the American government, is the ruling party. Helsinki Watch says that the party has targeted pensioners and villagers for intimidation. With one of the lowest birthrates in the world, the Bulgarian electorate is among the oldest in Europe.

An East European diplomat here said that whole villages have been told by party officials that "they will be punished" if the they do not vote as they are told.

"These are simple people who over the years have gotten used to seeing these kinds of threats come true," the diplomat said. "These are conservative people and whether we like it or not, the Communist Party presents stability. The party has said that it already has done the worst it can do and that the future can only be better."

Reform Communist leaders, oddly enough, insist that they do not want a big win.

"It would not be good if the party wins a very large victory. We need a coalition that will allow for good civilized fights, for the development of a political culture," said the adviser to President Mladenov.

The party has said that it will try to join the Union of Democratic Forces in a coalition government. Bulgaria, like many East European countries, is in deep economic trouble. It needs Western help to reschedule a $12 billion debt and rebuild its industrial base.

Without a coalition that includes the non-Communist opposition, party leaders say, Bulgaria has little chance of either attracting foreign assistance or controlling the emotions that have been released by the campaign.

The opposition, however, rejects the notion of a coalition.

"In a coalition with the Socialist Party we cannot take part," Zhelyu Zhelev, the opposition leader, said in a debate on Friday night. Such a coalition may be possible at some future stage, he said, but "it is not enough {for a Communist Party} to wake up and be a Socialist Party."

The differences between the two parties are primarily in the speed with which they would institute free-market change. The Union of Democratic Forces advocates a "quick, well-thought-out road to a market economy."

A number of internationally experienced election observers have come to Bulgaria to monitor the voting, and they suggest that the non-Communist opposition may do far better than opinion polls predict. They say that voters here were probably afraid to tell opinion pollsters which party they support. If people trust that their vote really is secret, these observers suggest, Bulgarians may overcome a long legacy of fear and surprise the Communists.