Democratic presidential nominee Joe Biden said Sunday he blames Donald Trump for "walking away" as COVID-19 ravaged the country, in a televised interview alongside running mate Kamala Harris on the eve of the Republican convention.

The president, who is trailing Biden in polls, has been heavily criticized for his response to the pandemic, during which he has pushed for lockdown restrictions to be lifted early, been hesitant to embrace face masks, touted treatments not supported by medical experts and insisted the virus will just "disappear."

"I don't blame him for the current crisis," Biden told ABC as the US neared 5.7 million infections and 177,000 deaths, by far the most in absolute terms of any country in the world. "I blame him for walking away and not dealing with solutions."

With the economy in shambles after pandemic-related lockdowns, Biden said again that the only way to get back on track was to bring the virus under control.

He vowed to heed scientific advice on the pandemic -- even if that would mean shutting the country down once more.

Biden slammed the segment of Americans who cite their "freedom" as they still resist wearing masks, despite evidence suggesting such barriers are one of the best ways to break transmission of COVID-19.

'Watch me, Mr President'

It's "the first time I've ever heard people say that doing something patriotic, you can save other people's lives, impacts on their freedom," he said.

The former vice president also responded to repeated attacks by Trump on his mental acuity, admitting that it was a "legitimate question to ask whether anybody over 70 years old" is fit for the presidency.

But the 77-year-old added: "Watch me, Mr President, watch me. Look at us both... what kind of shape we're in."

Questions have repeatedly been raised over 74-year-old Trump's own health, and he has previously claimed he "aced" a cognitive test that he said proves he has the mental stamina to lead.

The joint interview with Biden and Harris aired the night before the beginning of the Republican National Convention, during which Trump is seeking to recover momentum in his uphill re-election struggle.

Republicans were set to gather after the Democrats held their own convention last week, during which they accused the president of being a divisive and incompetent leader who, in the words of former first lady Michelle Obama, "cannot meet this moment."

During the interview, Harris, the first woman of color on a major party political ticket, also defended Biden's record on race as the US faces a reckoning over its treatment of black people and other minorities.

"Joe... actually knows how to say the words 'Black Lives Matter,'" Harris said.

Trump, in contrast, "has never spoken those words and will never speak those words," she said.