US President Donald Trump's testing a new tactic for when he's caught in a fib: blame advisers for giving him phoney information.

At a press conference on Thursday morning (Friday NZ time), Trump offered up several "alternative facts" about his voter support, including erroneous statistics and a brief rewrite of US election history.

For the third time in two days, he claimed to have won 306 electoral college votes, calling it "the biggest electoral college win since Ronald Reagan".

REUTERS US President Donald Trump was forced on the defence after being caught out on false claims about his voter support.

In fact, neither was true: Trump won 304 votes, and either way, he trailed his predecessor, Barack Obama, who won 334 electoral college votes in 2012 and 365 in 2008.

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Trump was asked: "why should Americans trust you when the information you provide is wrong?"

Trump: "I was given that information." pic.twitter.com/Og4zuuK2CO

— BuzzFeed News (@BuzzFeedNews) February 16, 2017

When journalists grilled him on the false figures, Trump said that he meant he achieved a bigger electoral college victory than any Republican since Reagan in 1980 and 1984.

Even that was wrong: as a reporter pointed out to the US President, another predecessor, George H . Bush, brought in 426 electoral votes in the 1988 election.

But the US President didn't back down or apologise for his erroneous claims, simply saying: "I was given that information. I don't know. I was just given it. We had a very big, big margin."

Asked why Americans should trust him when he gives out incorrect information, Trump offered no defence - instead blaming an adviser, before trying to suggest he was close enough to being correct.

"Well, I don't know. I was given that information," he said.

"Actually, I've seen that information around. It was a very substantial victory. Would you agree with that?"

From a historical viewpoint, Trump's electoral college margin over Democrat challenger Hillary Clinton rated at the slimmer end.

He ranked 45th out of 58 US presidential campaigns, dating back to George Washington's 1789 victory.

Trump needed 270 Electoral College votes to win the election, and easily succeeded, although Clinton won the popular vote by several million ballots.