Republicans are now outside mainstream politics - Obama
President Barack Obama has said that American politics has become "meaner" and that the rhetoric of Republican candidates such as Donald Trump and Ted Cruz has become "unrecognisable" to him.
Mr Obama has so far sought to remain aloof from the contest to succeed him but broke his silence in an interview in which he attacked the idea, espoused by Donald Trump, of banning Muslims from the United States.
He told 'Politico' magazine: "You think about it - when I ran (in 2008) against John McCain, he and I had real differences, sharp differences, but John McCain didn't deny climate science. John McCain didn't call for banning Muslims from the United States.
"The Republican vision has moved not just to the right, but has moved to a place that is unrecognisable."
Mr Obama added: "John McCain was a conservative but he was well within the mainstream of not just the Republican Party but within our political dialogue."
David Axelrod, Mr Obama's former senior strategist, writing in the 'New York Times', said: "Who among the Republicans is more the antithesis of Mr Obama than the trash-talking, authoritarian, give-no-quarter Mr Trump? His bombast allows no room for nuance or complexity."
Robert Gates, the former US Secretary of Defence, also criticised the level of debate among Republican presidential candidates.
He said: "I think middle-school kids would be embarrassed by the level of dialogue going on in the national campaign.
"These guys are making these broad pronouncements, it's clear they don't know what they're talking about.
"The worrying thing is they actually believe what they are saying - and if that is the case, we really are in trouble."
Also yesterday, Hillary Clinton attempted to fend off an insurgent left-wing challenge from Bernie Sanders for the Democratic presidential nomination, even while admitting that her favourite American president was a Republican.
She told a televised town hall meeting that Abraham Lincoln, the Republican who led the United States during the civil war and issued the proclamation freeing American slaves, was her chosen role model, overlooking her husband Bill's presidency.
Mrs Clinton said: "Sorry, President Obama, sorry, Bill - it's Abraham Lincoln.
"What could be more overwhelming than trying to win a civil war but he also kept his eye on the future and kept summoning up the better angels of our nature?"
Mrs Clinton was much less generous about present-day Republicans and issued a blistering moral condemnation of Mr Trump's call for a ban on Muslims entering the US when a Muslim woman wearing Islamic head-covering asked for an assurance that America was the best place to raise her three children.
"It's not only shameful and contrary to our values to say that people of a certain religion should never come to our country, it also dangerous," she said, without naming Mr Trump.
"We cannot tolerate this. We need a coalition that includes Muslim nations to defeat Isil and it's pretty hard to figure out how you're going to make a coalition with the very nations you need if you spend your time insulting their religion."
Several questioners told Mrs Clinton that they were leaning towards voting for Mr Sanders in next Monday's Iowa caucus, the first electoral test of the US primary season - with one even telling her that she was perceived as "dishonest".
Amid mounting evidence from the polls that she is losing support to Mr Sanders, Mrs Clinton - her party's putative frontrunner - told the audience in Des Moines, Iowa, that she was most qualified of the three Democratic candidates, who also include Martin O'Malley, a former Maryland governor, to become the next president.