By Steve Holland and Emily Stephenson

WASHINGTON (Reuters) - Rolling out an "America first" foreign policy, Republican front-runner Donald Trump vowed on Wednesday that if he were elected president, U.S. allies in Europe and Asia would have to fend for themselves if they did not pay more for the U.S. defense umbrella.

Trump's speech, delivered with a teleprompter in a staid Washington hotel ballroom, was an attempt to show he can be more presidential and move past the rancorous rhetoric that he routinely uses on the campaign trail.

"It is time to shake the rust off of America’s foreign policy," he said in a speech that savaged the foreign policy of Democratic President Barack Obama as a disaster.

The New York billionaire spoke the day after victories in five Northeastern states that moved him closer to capturing the Republican Party presidential nomination for the Nov. 8 election.

But the message Trump delivered appeared contradictory at times and was largely devoid of details.

He spoke of building up the U.S. military as a deterrent to U.S. adversaries, but said American allies in Europe and Asia would have to pay more for U.S. defenses provided by Washington.

While issuing that stern message on paying for defense, Trump said the United States under his leadership would be "a reliable friend and ally again."

Trump was sharply critical of immediate past presidents, both Republican and Democratic, for getting Americans involved in military conflicts abroad, but said the United States may well need to use force to defeat Islamic State militants.

In perhaps his most specific policy prescription, Trump said he would organize two summits, for NATO allies and Asian allies, to discuss "rebalancing" the alliances to ease the financial burden to the United States.

"The countries we defend must pay for the cost of this defense. If not, the U.S. must be prepared to let these countries defend themselves. We have no choice," Trump said.

Trump's speech was panned by his nearest rival for the Republican nomination, U.S. Senator Ted Cruz of Texas, who said in a tweet: "Donald's speech is the most dramatic evidence thus far that he fails the presidential test."

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Also critical was U.S. Senator Lindsey Graham of South Carolina, a hawkish Republican who was a presidential candidate until dropping out early this year.

“It’s isolationism surrounded by disconnected thought, demonstrates lack of understanding threats we face,” Graham wrote on Twitter. “Not sure who is advising Trump on foreign policy but I can understand why he’s not revealing their names."

Trump has gone from outsider last year to party front-runner with a plain-talking campaign that has often appealed to working-class voters who feel let down by globalization, free trade and the decline of American manufacturing in recent decades.

His foreign policy speech echoed that populist message - depicting a need to ease the U.S. financial burden overseas, focus more on nation-building at home and make sure American companies pay a price for outsourcing jobs to countries where labor is cheaper.

"My foreign policy will always put the interests of the American people, and American security, above all else. That will be the foundation of every decision that I will make," Trump said. "'America first' will be the major and overriding theme of my administration."

GERMANY REACTS

Niels Annen, foreign policy spokesman of the Social Democrat party group in the German parliament, criticized Trump's speech in a statement to Reuters.

"Trump‘s campaign gives America’s friends a feeling of uncertainty. At least he seems to have realized that as a candidate for the White House he would have to have a coherent foreign policy concept," Annen said.

Political pundits quickly pointed out that "America First" was a slogan first developed by famed aviator Charles Lindbergh, who pushed a non-interventionist policy before the U.S. entry in World War Two.

If Trump wins the nomination and runs against Democratic front-runner Hillary Clinton, he will be up against a former secretary of state steeped in foreign policy.

Clinton adviser Madeleine Albright, herself a former secretary of state, said Trump’s address was centered on “simplistic slogans and contradictions.”

"I’ve got to say, I’ve listened to a lot of foreign policy speeches over the years, and have given a lot myself, and I was hoping it would make some sense,” Albright said.

'SELLING A DREAM'

Jon Alterman, director of the Middle East program at the Center for Strategic and International Studies think tank in Washington, said Trump's speech did not contain enough details to call it a strategy.

"Ultimately, he’s selling a dream and he’s still not offering a plan. He’s representing the sales office, but he’s given no clue of who will be the architect and who will do the construction," he said.

Trump did not stray from the ideas that have helped put him close to winning the Republican presidential nomination. He would build up the U.S. military but would use it only when necessary and would be unpredictable in using force, as in the case of Islamic State.

"Their days are numbered. I won’t tell them where and I won’t tell them how. We must, as a nation, be strong and resilient. They’re going to be gone," he said.

Trump said U.S. leaders had allowed the American nuclear arsenal to atrophy and that it was in desperate need of modernization. The Obama administration is pushing a $1 trillion refurbishment program over the next 30 years.

Trump was particularly withering in his critique of Obama's foreign policy, saying the president had let China take advantage of the United States and not been able to persuade Beijing to rein in North Korea.

"We have the leverage. We have the power over China, economic power," he said.

According to social media analytics firm Zoomph, Twitter reaction to the speech was more positive than negative, with roughly 100 posts per minute mentioning the Republican front-runner.

Walid Phares, a Trump foreign policy adviser, said Trump received input for his address from a variety of advisers, such as U.S. Senator Jeff Sessions of Alabama.

(Additional reporting by Arshad Mohammed, David Brunnstrom, Amanda Becker, Matt Spetalnick, Warren Strobel and Richard Cowan, Susan Heavey and Doina Chiacu in Washington and Anjali Athavaley and Melissa Fares in New York; Editing by Frances Kerry and Peter Cooney)