Czarek Sokolowski, Associated Press
Polish Prime Minister Jaroslaw Kaczynski, right, hands resignation papers to his twin, President Lech Kaczynski.

WARSAW, Poland — For nearly 16 months, Poland had perhaps the world's most unusual leadership — identical twin brothers as president and prime minister with the same round faces, crops of silver hair and defiantly patriotic views.

That twin act came to an end Monday when Jaroslaw Kaczynski resigned as prime minister in the wake of a bruising electoral defeat, paving the way for a new government that wants Polish troops out of Iraq and promises a better bargain in exchange for hosting a proposed U.S. missile defense shield.

Kaczynski's resignation leaves his brother, President Lech Kaczynski, to face three more years in office without the luxury of a prime minister to whom he is bound by the ties of blood, deep affection and a shared political philosophy.

Jaroslaw Kaczynski's socially conservative and nationalist Law and Justice party was ousted by voters in parliamentary elections on Oct. 21. The victor was Civic Platform, a pro-business party that seeks to improve ties with the European Union and vows to demand more from its alliance with the United States.

The president — whose term runs until 2010 — must now ask the newly elected Donald Tusk to form the next government, a step planned for later in the week.

Dressed in black, Jaroslaw Kaczynski showed up at the presidential palace with his Cabinet ministers. Addressing his brother as "Mr. President," he offered up his government's resignation and summed up what he said were its achievements.

Among them, he mentioned a sharp drop in unemployment and the strong economic growth this ex-communist country has seen under his tenure.

"We can finish our term with our head held high," Jaroslaw Kaczynski said. "The results, not just in words but in reality, are good."

Later, he addressed lawmakers as the newly elected parliament met for the first time, and argued in support of hosting a U.S. missile defense site.

Poland has been in negotiations for months with Washington on a U.S. proposal to host 10 interceptors that would be part of a global shield meant to defend the United States and its allies from long-range missile strikes from Iran and North Korea.

Tusk has expressed irritation over what he says is far too little reward for Poland's support of the U.S. in Iraq and Afghanistan. Civic Platform vows to drive a harder bargain with the United States in return for hosting the missile defense base and has promised to withdraw Poland's 900 troops from Iraq next year, though no date has been set.

Jaroslaw Kaczynski told lawmakers the United States was the only country "in a position to lend us effective help."

"Strengthening that alliance, even in its difficult aspects — and it is not an easy ally — is without a doubt in the national interest," Kaczynski said. "Yielding to those who don't want the shield ... would go against our national interest."

A resurgent Russia — the region's overlord during the Cold War — is furious about U.S. plans to place missile defense bases in Poland and in the Czech Republic, and has warned that such moves could lead to a new arms race.