U.S. President Barack Obama on Monday laid some of the blame for the tone of the presidential campaign on political journalism that has been pinched by shrinking newsroom budgets and cheapened by a focus on retweets and likes on social media.

In a speech to a journalism awards dinner, Obama urged journalists to ask tougher questions of the candidates vying to be president. He voiced dismay over the vulgar rhetoric, violence at rallies and unrealistic campaign pledges that have continually grabbed headlines, in a thinly veiled reference to Republican front-runner Donald Trump.

He said the media landscape has changed since his first presidential campaign in 2008, when "there was a price if you said one thing and then did something completely different."

The New York Times earlier this month reported that Trump has so far earned almost US$1.9 billion worth of media coverage, compared with US$313 million for the next closest Republican challenger, U.S. Senator Ted Cruz of Texas, and US$746 million for Democratic front-runner Hillary Clinton.