WASHINGTON – America supports expanding NATO to include Ukraine, a former Soviet republic now trying to loosen historic ties to Russia, but membership in the Western alliance is not guaranteed, President Bush said yesterday.

“There is a way forward in order to become a partner of the United States and other nations in NATO,” Mr. Bush said during a joint press conference with Viktor Yushchenko, the populist politician whose Orange Revolution forced out Ukraine’s pro-Russian government last year.

“It’s not a given. In other words, there are things that the Ukrainian government must do,” Mr. Bush said.

NATO membership is by invitation of the member states, and requires guarantees of political, military, and economic openness. For Ukraine, joining NATO would mean taking more decisive steps away from Russian influence and cleaning up systemic corruption.

“We want to help your government make the difficult decisions and difficult choices necessary to become available for membership in NATO,” Mr. Bush said.

“The ideals for the new Ukraine are the ideals shared by Western civilization,” Mr. Yushchenko said through an interpreter.

Mr. Yushchenko called corruption the No. 1 problem at home. He has promised a thorough investigation of corruption and alleged political skullduggery during his predecessor’s 10 years as president.

Mr. Yushchenko later spoke to the largest American business lobbying group, the U.S. Chamber of Commerce, and sought to assure business leaders that Ukraine is trying to fight corruption and attract investments.

“I would like to clearly state that the rules of the game [have] changed in Ukraine, that the law is working in Ukraine,” he said through an interpreter. “From now on the Ukrainian state, the Ukrainian government are going to protect your interest.”

He asked business leaders “please, do not give any bribes in Ukraine to anybody,” saying he hoped that by cutting bribes from their budgets, they would save enough money to become more profitable.

Mr. Yushchenko is on a three-day trip to America to lobby for aid and investment, win Washington’s support for joining NATO, and greet Ukrainian-Americans. He will address Congress later in the week. The trip comes a little more than two months after Mr. Yushchenko took office following a dramatic popular uprising. Masses of supporters camped out in Kiev, claiming that a Kremlin-backed candidate stole a disputed election. The government was forced to allow a second vote, which Mr. Yushchenko won.

After the election, Mr. Yushchenko claimed that the Russian-backed regime of his predecessor, Leonid Kuchma, had tried to assassinate him. Mr. Yushchenko suffered near-fatal dioxin poisoning during last fall’s presidential election, which left his once smooth face sallow and pocked.

Mr. Bush frequently points to Mr. Yushchenko’s peaceful rise as an example of the power of democratic ideals. Mr. Bush and Secretary of State Rice have said that advancing Ukrainian style democratic change is the unifying theme of Mr. Bush’s second term.

Yesterday, Mr. Bush called Mr. Yushchenko “an inspiration to all who love liberty,” and said Mr. Yushchenko was the first world leader he called after his inauguration for a second term in January.

The two leaders stepped quickly past the most visible irritant in their new friendship. Mr. Yushchenko is withdrawing Ukraine’s troops from Iraq, a campaign promise that acknowledges the deep unpopularity of the Iraq deployment among Ukrainians.