(CNN) Franklin D. Roosevelt sent vast armies across the Atlantic. John F. Kennedy thumbed his nose at communism in Berlin. And Ronald Reagan chose the same iconic city to call on Mr. Gorbachev to "tear down this wall."

President Barack Obama's chance to save Europe comes on Friday in less dramatic surroundings: at a London news conference with British Prime Minister David Cameron. But it could be a signal moment in the continent's modern history nonetheless.

That's because Obama has traveled to London to deliver a message to British voters that they should reject walking out on the European Union at a referendum in June -- because the future prosperity and security of the West is once again on the line.

Obama was polite in London, seeking to avoid further inflaming a political conflagration over the referendum that has prominent members of the "Leave" campaign branding him a hypocrite who is interfering in British affairs.

"Let me be clear, ultimately this is something that the British voters have to decide for themselves," Obama said alongside Cameron at a news conference in London. "But as part of our special relationship, part of being friends, is to be honest and to let you know what I think."

He did not tell U.K. voters outright what to do, but he did not hide his view that a British exit -- dubbed "Brexit" -- is a very bad idea.

"The United Kingdom is at its best when it is helping to lead a strong Europe," Obama said. "It leverages UK power to be part of the European Union."

Obama also framed his position in an op-ed for The Telegraph published Friday, praising the accomplishments of the European Union as well as pointing out that the EU benefits from British influence.

"The United States sees how your powerful voice in Europe ensures that Europe takes a strong stance in the world, and keeps the EU open, outward looking, and closely linked to its allies on the other side of the Atlantic," Obama wrote, addressing U.K. citizens. "So the US and the world need your outsized influence to continue -- including within Europe."

Obama made sure to point out that "ultimately, the question of whether or not the UK remains a part of the EU is a matter for British voters to decide for yourselves."

Washington fears that a decision to abandon the European project, in which Britain has been an often-reluctant participant since 1973, would threaten to unravel the union, a vital ally, as it struggles with disunity and unleash a new instability in an already dangerously volatile world.

While advocates of a British departure say the country will be stronger freed from the shackles of Brussels bureaucracy, opponents fear it could dismember the U.K. should Scotland decide its future still lies in the E.U. and demand a fresh independence vote.

While there are many issues that Obama needs to address in London -- including Syria, Europe's refugee crisis, Russia's military provocations, terrorism and ISIS -- the timing of his visit before the referendum is intended as a clear signal.

"The principal policy objective of President Obama coming to the U.K. at this time is to lay out the American perspective on the U.K. in Europe," said Xenia Wickett, head of the U.S. and Americas program at Chatham House, a top foreign policy think tank in London, and who has served on the National Security Council and at the State Department.

There's some irony to Obama's appeal. After all, he's a man of the Pacific who's often seemed more temperamentally and strategically drawn to the dynamic future of Asia than Europe's glorious past. He has been less influenced by an affinity with Europe than generations of previous American statesmen and has appeared cooler to the "special relationship" than most recent presidents.

And throughout his presidency, Obama has expressed irritation at the neediness of European leaders competing for the attention of the American president. He's grumbled that European nations won't do more for NATO and even publicly rebuked Cameron in a recent Atlantic magazine interview for not doing more to bring peace to Libya after a NATO intervention in 2011 pushed by him and other Europeans.

In fact, Obama's most memorable moment in Europe so far may have come before he was even elected president, when in 2008 several hundred thousand Berliners listened to his intoxicating vision of hope and change, setting up expectations at home and abroad that he struggled to meet once he moved into the White House.

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Obama, who has had plenty on his own political plate, hasn't developed a sudden urge to meddle in Britain's politics or an altruistic concern for its people. But the prospect of a new European crisis precipitated by a British exit from the bloc is concentrating minds in the White House.

His trip is also an opportunity to remind Americans of the dividends a close relationship with Europe pays the U.S. in terms of keeping global peace and stretching U.S. power and influence, after Republican presidential front-runner Donald Trump questioned the use and cost of American allies across the Atlantic.

The unexpectedly tight state of the referendum race -- with only a narrow majority of Britons in recent polls backing continued E.U. membership -- means anything Obama does say has the potential to be highly controversial.

London Mayor and cabinet minister Boris Johnson, a potential future Conservative prime minister, accused Obama of "outrageous and exorbitant hypocrisy."

The flamboyant Johnson, who has broken with Cameron on the referendum, argued that the U.S. president would never accept the kinds of constraints on American sovereignty and that E.U. membership demands of Britain, a point he reiterated in an oped for the Sun published Thursday.

"It is incoherent. It is inconsistent, and yes it is downright hypocritical," Johnson wrote . "The Americans would never contemplate anything like the EU, for themselves or for their neighbours in their own hemisphere. Why should they think it right for us?"

And Nigel Farage, leader of the pro-exit United Kingdom Independence Party, branded Obama a "very anti-British American president" in an interview with ITV News.

"I don't think what Obama says will make any difference to public opinion at all."

But Obama feels compelled to walk the Brexit tightrope despite the criticism because the stakes are so high for the U.S.

A British departure could prompt other nations to follow suit and further fracture the continent's unity. It could also have unpredictable consequences for the global economy and financial markets. And it would come at a time when Russia is emulating its Cold War military adventurism and ISIS terrorists are threatening Europe.

If Scotland were to seek to quit the U.K. to rejoin the E.U., it would cause headaches for NATO since the U.K.'s nuclear deterrent operates out of a submarine base northwest of Glasgow that nationalists have pledged to close.C

Obama's top aides have also hinted that a British withdrawal from the bloc could render the U.K. a less valuable ally.

Photos: The Obamas meet British royalty Photos: The Obamas meet British royalty President Barack Obama, Prince William and first lady Michelle Obama talk with Prince George at Kensington Palace on April 22, 2016, in London. Hide Caption 1 of 10 Photos: The Obamas meet British royalty Obama and the first lady are greeted by Queen Elizabeth II and Prince Phillip after landing by helicopter at Windsor Castle for a private lunch on April 22, 2016. Hide Caption 2 of 10 Photos: The Obamas meet British royalty Philip drives the President, the first lady and Elizabeth from the helicopter into Windsor Castle after the Obama's arrived for a private lunch in Windsor, England, on April 22, 2016. Hide Caption 3 of 10 Photos: The Obamas meet British royalty Elizabeth, the President, the first lady and Philip pose for a photograph in the Oak Room ahead of a private lunch at Windsor Castle on April 22, 2016. Hide Caption 4 of 10 Photos: The Obamas meet British royalty The President, the first lady and the Duke and Duchess of Cambridge meet in London's Buckingham Palace on May 24, 2011. Hide Caption 5 of 10 Photos: The Obamas meet British royalty President Barack Obama meets with Prince Harry in the Oval Office on October 28, 2015. Hide Caption 6 of 10 Photos: The Obamas meet British royalty Queen Elizabeth II poses with the President, the first lady and Prince Philip, Duke of Edinburgh, in the Music Room of Buckingham Palace ahead of a State Banquet on May 24, 2011. Hide Caption 7 of 10 Photos: The Obamas meet British royalty Jill Biden, Prince Harry and Michelle Obama at the Joining Forces Invictus Games 2016 Event at the Wells Fields House on October 28, 2015, in Fort Belvoir, Va. Hide Caption 8 of 10 Photos: The Obamas meet British royalty President Barack Obama sees off Queen Elizabeth II, as the Duke of Edinburgh and the first lady follow after a reciprocal dinner at the Winfield House in London, on May 25, 2011. Hide Caption 9 of 10 Photos: The Obamas meet British royalty Queen Elizabeth II speaks with President Barack Obama during a group photo of world leaders attending the D-Day 70th Anniversary ceremonies at Chateau de Benouville in Benouville, France, on June 6, 2014. Hide Caption 10 of 10

"For many decades, the United States had benefitted greatly from the role that the United Kingdom plays in the world, a truly unique role as a leader on behalf of not just security and prosperity but democratic values," said Ben Rhodes, a deputy national security advisor.

"Part of the way in which the U.K. has exercised that leadership is through the European Union," Rhodes said, clearly stating America's preference for a close ally that is often on the same page as Washington, at the center of the bloc.

For British officials, the suspicion is that Washington could see its old ally as less worthy of its cherished "special relationship" status, a concern that has already caused handwringing across the Atlantic at earlier points in Obama's tenure.

Anxiety about the possibility of a British exit from the E.U. is not just troubling 10 Downing Street and the White House but the broader foreign policy establishment in Washington.

"We don't want to go into these unknowns at a moment of global instability -- in the Middle East, in Europe, in Asia, with Russia," said Heather Conley, of the Center for Strategic and International Studies in Washington.

Some Republicans, however, are criticizing Obama for his intervention.

Senate Foreign Committee Chairman Bob Corker, a Tennessee Republican, told CNN that he did not support telling the British people how to vote: "I haven't appreciated foreign efforts to influence political decisions in our country, so I do not support American attempts to do the same in the United Kingdom."

Clearly, there are no qualms at 10 Downing Street about his impact -- he and Cameron will appear together just in time for the early evening television news and Obama is certain to hijack the media cycle for several days. It may be no coincidence that the President's former re-election campaign manager Jim Messina is consulting with Cameron on the referendum campaign.

Any trip to Britain by a U.S. president -- and the paraphenalia he brings with him: Air Force One, fleets of helicopters and motorcades of armored limousines -- get blanket coverage.

Obama also still enjoys in the U.K. the kind of star power that has ebbed back home, and his positions on issues like gun control, health care and military restraint are a good fit with Britain's electorate, offering him added political credibility.

It can't hurt either that he will use his visit to show respect for Britain by lunching with Queen Elizabeth II at Windsor Castle to mark her 90th birthday.

"His voice will definitely have resonance," said Nicholas Dungan, and specialist in transatlantic relations at Sciences Po, one of France's top schools on international relations.

"If there is one place in Western Europe where the opinion and policy of the U.S. is taken really seriously, it is Britain," said Dungan, speaking from London. "It is a watershed in the debate."