Image copyright AP Image caption Mr Obama has met Ed Miliband before - but not at the White House

How do you look more prime ministerial?

That's a question that faces any leader of the opposition. One answer is for people to see him doing the things which prime ministers do - like meeting the President of the United States.

Sources on both sides of the Atlantic have told me that Ed Miliband has been lobbying hard for a meeting with President Obama.

It will have to be arranged soon to avoid the run-up to the Scottish referendum and this autumn's American mid-term elections. After that, the White House fears that it will be seen to be interfering in the British general election campaign.

Jonathan Powell, who arranged the last visit to the White House by a Labour leader of the opposition - with Tony Blair way back in 1996, told me it is "the nearest the leader of the opposition gets to a job interview for prime minister".

"It's when he turns up and it has to go right but if it goes wrong there are very serious consequences back home," he says. "Of course, Neil Kinnock proved that".

Kinnock and Reagan

Labour has long been haunted by the memory of their leader's calculated humiliation by President Reagan in the run-up to the 1987 general election.

Neil Kinnock was granted half an hour of the president's time but the meeting didn't go to plan.

After it ended, the Labour team proclaimed how well it had gone. Soon after the White House issued a statement expressing their deep concerns about Labour's policy of nuclear disarmament.

Weeks later Reagan's friend, Margaret Thatcher, was re-elected.

This time will be different, won't it? As President Obama's a Democrat surely he'll get on just fine with Labour's Ed Miliband?

After all, Labour's election campaign is being masterminded by the man Obama calls the Axe - his former adviser David Axelrod.

But American sources have told me that Mr Miliband's foreign policy stance has not endeared him to the White House - he opposed their position on Palestine at the United Nations and first backed military action in Syria then opposed it.

'Syrian bruises'

PJ Crowley, who worked in the Obama administration as Assistant Secretary of State at the US State Department, says there are "some bruises" from how the Syrian crisis played out.

"The rejection in the Houses of Commons did have ripple effects here in terms of how the President confronted the prospect of a military strike against Syria. "I think it did alter the President's strategy in Syria."

Image copyright AP Image caption Tony Blair was the last opposition leader to meet a US President

But Mr Crowley believes it will be the president's schedule, not his personal opinions, which will decide whether this meeting ever takes place.

"The relationship between the United States and Britain is too important to let it get bogged down by pique and personality but there is only one president and to an extent Mr Miliband may be seeking a guarantee that if he comes to Washington he will be able to see the president," he says.

"(But) world events do intercede."

In other words, if Ed Miliband gets a date he should be ready to see it cancelled.

Now a meeting at the White House is not always quite a meeting. The leader of the opposition will probably get what's known in the trade as a brush by.

When David Cameron was leader of the opposition he had a meeting at the White House with President Bush's National Security Adviser until George W just happened to brush by.

So much to worry about...so much that can go wrong….as Gordon Brown found at his frequently awkward meetings with President Obama.

'Be prepared'

Again and again Gordon Brown had to bat off questions about whether he'd been snubbed by the leader of the free world.

Reporters who travelled with him to the UN remember a brush by with Barack Obama in the kitchens or what The Sun dubbed Gordon's kitchen nightmare.

Labour is desperate not to ruin Ed Miliband's chances of a visit to the White House by talking them up.

Jonathan Powell's has this advice for them.

"My advice under any circumstances would be don't do it unless you can be absolutely sure it is going to go right," he says. "We spent our time making absolutely sure we were prepared. What you should never do is go ahead on the hope it will turn out well."

The Labour leader's aides point out that he has already met Barack Obama, though not at the White House.

Some even point out that Michael Howard never got to go when he was Tory leader - before they're reminded it is, perhaps, not the best parallel to draw.

Don't be fooled though. In Washington DC they know that this is a meeting the man who aspires to be our next prime minister wants and wants very badly.