President Barack Obama has moved to discredit Donald Trump suggesting he would be exposed as unfit to serve in the White House if reporters probed him more urgently.

“I just want to emphasize the degree to which we are in serious times and this is a really serious job,” Mr Obama told the White House press corps on Friday. “This is not entertainment. This is not a reality show. This is a contest for the presidency of the United States.”

Making his first remarks on the Republican race since Mr Trump emerged as the presumptive nominee, Mr Obama told the reporters that deeper reporting of the billionaire and his positions had now become paramount, implicitly criticising them for have fallen short thus far. “Emphasizing the spectacle and the circus, that’s not something we can afford,” he said.

Barack Obama will gauge the federal government's role in Flint's recovery <em>Alex Wong/Getty</em>

“It's important for us to take seriously the statements he's made in the past,“ Mr Obama went on. “Every candidate, every nominee has to be subjected to exacting stands and genuine scrutiny,”he said. That includes making sure their proposed budgets add up. “If they don’t, that needs to be reported on and the American people need to know that,” he suggested.

“If they take a position on international issues that could threaten war, or has the potential of upending our critical relationships with other countries, or would potentially break the financial system, that needs to be reported on,“ he went on, clearly referencing some foreign policy remarks made by Mr Trump, including suggesting Japan and South Korea acquire nuclear weapons.

At the White House Correspondents’ Dinner last weekend, Mr Obama offered serial cutting jokes about the billionaire businessman. But on Friday he was stern, dismissing out of hand a question about a now infamous Tweet from Mr Trump on Thursday, which was a holiday in Mexico, showing him eating a so-called Taco Bowl salad and declaring his love for Hispanics.

“I have no thoughts on Mr Trump’s Tweets,” he replied. “As a general rule I don’t pay attention to Mr Trump’s tweets.” Nor would he he said, for the next six months until the general election.

Prodded to comment on the schism that MrTrump’s ascendancy has opened in the Republican Party and particularly the refusal of the House speaker Paul Ryan to endorse him, Mr Obama was circumspect, saying he’d leave it up to the Republicans to figure out how to “square their circle”. But he conceded: “There is no doubt that there is a debate that's taking place inside the Republican Party about who they are and what they represent.”

But he seemed to betray confidence that ordinary Republicans, in the end, would see through the billionaire businessman and former reality TV host.

“Voters are going to have to take a decision if this is the guy that speaks for them and represents their values,” he said. “Republican women voters are going to have to decide if that is the I feel comfortable with in representing me and what I care about.”

Already Mr Trump is struggling to bring the Republican Party behind him, with several key figures declaring they are unable to offer their support, including two former Republican presidents - George H.W. Bush and his son, George W. Bush - and the 2012 nominee of the party, Mitt Romney, who has said he will now stay away from the party’s July convention

How much these defections - and Mr Ryan’s reluctance to embrace him - will harm Mr Trump is hard to calculate. He managed to corral so much support during the primary races in part because voters are entirely disgusted with the party establishment and the leaders in Washington, of whom Mr Ryan is clearly one.

Attacks from Mr Obama, meanwhile, are likely to be worn by Mr Trump as a badge of honour.

Mr Obama had come to the press room in the first place to herald a new tax rule unveiled by the US Treasury on Friday that will force banks and investment houses to reveal and report the real owners of companies they serve, in a bid to stop the setting up of shell companies of the purpose of tax evasion or fraud.

“These actions are going to make a difference,“ Mr Obama said, adding that they would help the US authorities to make people are ”paying the taxes they owe rather than using shell corporations and offshore accounts to avoid doing the things that ordinary Americans are doing every day.”