American plans to build a missile defense shield in Eastern Europe to intercept an attack from Iran may be put in jeopardy if, as expected, the leader of the pro-business Civic Platform party becomes Poland’s new prime minister today.

Early indications of the result of the snap Polish general election yesterday showed the pro-business, pro-European Union, socially conservative Donald Tusk, 50, heading for victory, ousting Prime Minister Jaroslaw Kaczynski, who, with his identical twin brother, President Lech Kaczynski, has governed the country in a fragile coalition with two minor parties for the last two years.

During the campaign, Mr. Tusk said he thought Poland’s deal to allow America to place 10 interceptor missiles in his country was struck too cheaply. He has proposed in exchange a raft of business deals with American companies and the introduction of a visa waiver system that would allow Poles to travel to America unhindered.

In an election debate with the outgoing prime minister on October 12, Mr. Tusk hinted that he would renegotiate the defense shield pact with America.

“You say we’ve gained a lot from our ties with the U.S., but you don’t name a thing. Where are the visas? Where are the business deals?” Mr. Tusk asked Mr. Kaczynski. Mr. Tusk’s robust performance during the debate led to a surge in the opinion polls and all but assured him victory in yesterday’s poll.

It is understood that Mr. Tusk also will demand from America additional security guarantees for Poland, including the provision of Patriot short-range anti-missile and anti-aircraft missiles. Russia has expressed disquiet at the missile shield being so close to its borders and has made threatening noises against Poland and Czechoslovakia, where the shield is to be sited.

“Poland must have a security guarantee tied to basing a missile shield on its soil. I’m talking about a missile defense system against short-range missiles, such as Patriot missiles,” a leading Civic Platform lawmaker, Slawomir Nowak, told the Associated Press.

The Civic Platform Party platform also demands the early return of Poland’s military contingent in Iraq, by the end of next year. Prime Minister Kaczynski was a devoted ally in the American-led war on terror and twice extended the Polish presence in Iraq. Mr. Tusk also will consider whether to withdraw the 1,000 Polish troops serving in Afghanistan. Although Mr. Tusk has said he would like more from the Americans before finally agreeing to the interceptor missile base, he is avidly pro-American and pro-Western and is unlikely to press the bargain to the limit.

“We will pursue very friendly ties with the United States. The U.S. is our greatest ally, our biggest partner, biggest friend, and will remain all those things. We count and depend on our strategic relationship with the U.S.,” Mr. Nowak said.

Mr. Tusk promises to introduce a raft of Thatcherite economic policies to Poland, with lower taxes, fewer regulations on business, less bureaucracy, and the privatization of state assets. The first measure is expected to be a 15% flat tax on all goods, income, and businesses.

The new government is aiming to lure home the 1.5 million Poles who have left the country since the fall of communism in 1989. Since then, most Poles have expatriated themselves to Britain and Ireland, where they are entitled to work because of the European Union’s free market in labor.

Exit polling yesterday showed the Civic Platform with 43%–45% and the Kaczynskis’ Law and Justice Party trailing with 30%–33%. Mr. Tusk is expected to form a coalition, perhaps with the Peasants’ Party, which will likely poll 8%, in order to win a majority in Poland’s 460-member parliament.

The election is a vindication for Mr. Tusk, a former dissident against communism and father of two who was dismissed by government figures during the election as a lightweight. The campaign was colored by a succession of dirty tricks, notably from Law and Justice, including a “sting” in which a parliament member was photographed accepting a $100,000 bribe.

Smears were in evidence in the presidential election two years ago, when on the eve of the poll Lech Kaczynski charged that Mr. Tusk’s grandfather was a member of the German Wehrmacht during World War II. While true, the smear concealed the fact that as a German citizen, Mr. Tusk’s grandfather was obliged to join the German forces, though 10 days later he deserted to join the Free Polish Forces in the West.

Last week’s televised debate proved to be a key event, with Mr. Tusk repeatedly accusing Mr. Kaczynski of proving inadequate to the task and his economic policies responsible for the emigration of young Poles to more affluent countries in the European Union. Until then, the two leaders were running neck and neck in the polling.