WASHINGTON — As Senator Obama basks in the adulation of more than 200,000 cheering Europeans in Berlin, Republicans are banking on a backlash at home.

Introducing himself as a “fellow citizen of the world,” the presumptive Democratic nominee stood in the German capital and called for Europe to stand with America in the fight against terrorism and forge a united front to eliminate nuclear weapons and curb the damage wrought by global climate change.

“In this new century, Americans and Europeans alike will be required to do more — not less,” Mr. Obama said. “Partnership and cooperation among nations is not a choice; it is the one way, the only way, to protect our common security and advance our common humanity.”

In a much-anticipated speech on foreign soil, Mr. Obama spoke near the Victory Column at Tiergarten Park, not far from where the Berlin Wall once stood. A crowd that German police officials estimated exceeded 200,000 interrupted Mr. Obama with repeated ovations, particularly as he evoked President Reagan in calling for a rejuvenated alliance between America and the nations of Europe that would tear down the walls of animosity and intolerance.

“The walls between old allies on either side of the Atlantic cannot stand,” he said. “The walls between the countries with the most and those with the least cannot stand. The walls between races and tribes; natives and immigrants; Christian and Muslim and Jew cannot stand. These now are the walls we must tear down.”

While the speech was clearly well-received in Germany, the campaign of Mr. Obama’s rival for the presidency, Senator McCain, said it smacked of arrogance and catered to the wrong audience. “While Barack Obama took a premature victory lap today in the heart of Berlin, proclaiming himself a ‘citizen of the world,’ John McCain continued to make his case to the American citizens who will decide this election,” a McCain spokesman, Tucker Bounds, said in a statement.

The Arizona senator tended to domestic concerns in America, campaigning in the crucial battleground of Ohio. In a nod to the Obama speech, he stopped at a German restaurant in Columbus, where he said that while he’d love to give a speech in Germany, “I’d much prefer to do it as president of the United States, rather than as a candidate for president.”

The McCain campaign appears resigned to the fact that Mr. Obama’s foreign trip is dominating the political news this week and that the Illinois senator has thus far avoided the kind of game-changing gaffe that Republicans were hoping to exploit. Instead, Mr. McCain’s aides are trying to feed a perception that Mr. Obama is overreaching as he tries to bolster his foreign policy bona fides and present himself as a credible world leader.

They seized on a report yesterday on the Web site of the Atlantic Monthly that Mr. Obama had begun assembling a transition team for after the election. And a senior foreign policy adviser, Randy Scheunemann, pointed to a flourish in Mr. Obama’s speech — “People of Berlin, people of the world: This is our moment. This is our time.” — that was nearly identical to a climactic line of the speech he delivered in Minnesota last month on the night he clinched the Democratic nomination.

“Now he seems to be setting himself up as president of the world, not just citizen of the world,” Mr. Scheunemann said in an interview. “We certainly wouldn’t use language like that.”

Political and foreign policy analysts acknowledged that Mr. Obama risked creating the impression that he was cozying up too much to the European community, which some American voters view warily. And his extended trip abroad comes as he has yet to build the kind of commanding lead in the polls that some political observers had predicted, given the level of enthusiasm surrounding his candidacy. New surveys from Quinnipiac University released yesterday showed that Mr. McCain was closing the gap in four key states, including Michigan, Minnesota, and Colorado.

“It is in a sense pretty presumptuous,” a political scientist at Franklin and Marshall College in Pennsylvania, G. Terry Madonna, said. But he added: “I don’t think there’s going to be a backlash in the United States.”

The political climate is different than in 2004, he said, when Republicans succeeded in characterizing Senator Kerry as overly sensitive to the opinion of American allies in Europe. “In 2004, we didn’t have a recession. The Iraq war wasn’t as unpopular,” Mr. Madonna said.