Joe Biden has accused Donald Trump of being "behind the curve" leading the US response to the coronavirus pandemic after the president berated reporters and wavered on emergency actions to rush potentially life-saving medical supplies to health workers.

The Democratic presidential candidate told reporters on Friday that "the American people deserve a president who tells them the truth ... Unfortunately, President Trump has not been that president."

Mr Biden's message follows the president's dismissal of a question about what to tell frightened Americans during the crisis during a White House press conference. The president responded by calling the journalist who asked a "terrible reporter."

The former vice president said: "People are scared. They're worried. They don't know quite what to do."

He told the president: "Step up and do your job."

The race for the Democratic nomination to face the incumbent in November has whittled down significantly not just in the field of contenders but in the scope of their respective campaigns as the candidates develop their response to the Covid-19 crisis.

Bernie Sanders, the former vice president's rival in the 2020 race, has addressed supporters in frequent "fireside chat" livestreams and addresses that he broadcasts from Vermont to discuss the emergency and plans to bolster federal relief legislation in the Senate.

The Vermont senator has argued that the coronavirus and the White House response have exposed systemic weaknesses throughout the US, underlining his call for significant government reform.

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Their responses to the coronavirus — and messages to Americans in attempts to offer some comfort — stand in stark contrast to the president's wildly inconsistent messaging and often-inflated claims about his administration's success.

On Thursday, Tulsi Gabbard, who clung to her campaign despite significant losses in primary states thus far, withdrew from the race and endorsed Mr Biden. The Hawaii congresswoman said she is better suited during the "unprecedented global crisis" to stay in Congress and "to stand ready to serve in uniform should the Hawaii National Guard be activated."

Though he did outline a plan to combat the outbreak and offered it to the White House, Mr Biden has dodged criticism for fading from the public after debates and primary elections in the wake of the national emergency.

He said he wants to be in "daily" or "significant" contact with Americans throughout the crisis.

In his remarks to reporters on Friday, he also attacked Mr Trump's hazy invocation of the Defence Production Act, which would allow the president to force companies to manufacture critically needed medical supplies, like ventilators and other equipment.

He also criticised the administration's previous calls for state governors to order equipment themselves: "He said the federal government is not a shipping clerk. Where the hell did that come from?"

The president issued an order that invokes the law, but later said that he would only use it during a "worst case scenario" and appeared to renege on previously announced plans. On Friday, the president said he has put the law into "high gear" though didn't offer specifics.