"I am calling on Biden to stop promoting his anti-vaccine theories," he said at a White House briefing, minutes before claiming without evidence that a vaccine will be distributed "sometime in October" and contradicting his own CDC chief, who testified under oath that it will not be widely available until well into 2021.

Polls show growing public skepticism about a coronavirus vaccine, and if millions of people shun a shot, it won't bring an end to the pandemic. So Biden tried to thread the needle Wednesday between trust in vaccines and distrust in a Trump vaccine, unless it is validated by nonpolitical, transparent science.

Biden's speech laid out the criteria he would use to evaluate a vaccine if the Trump administration approves one before the end of the year, and called for "total transparency" from the drug companies developing the vaccine and the rank and file scientists at the FDA and CDC so that the public can vet political officials' claims.

"What criteria will be used to ensure that a vaccine meets the scientific standard of safety and effectiveness?" he said. "Second, if the administration green lights a vaccine, who will validate that the decision was driven by science rather than politics? What group of scientists will that be? Thirdly, how can we be sure that the distribution of the vaccine will take place safely, cost-free, and without a hint of favoritism?"

If all those criteria were met, he added, he would absolutely take the vaccine himself and encourage others to do so as well. But without that information, he warned that the president's word won't be enough to alleviate widespread skepticism about the vaccine.

"The American people right now don't trust what the president says about things relating to science," he said, citing recent Trump remarks questioning the science on wildfires and climate change as well as claims the virus will "go away" with or without a vaccine.

Republicans have recently pounced on statements by Biden, his running mate Kamala Harris and other Democrats about vaccines, accusing them of promoting "anti-vaxxer" talking points. Yet new polling finds Republicans are far less likely to say they'll take a vaccine when it's available than Democrats, and a majority of voters say they do not trust Trump's claims about a vaccine.

Biden also pointed to the extensive work his own team is undertaking so that they will be ready to take over a national vaccination campaign in January if he wins in November, including creating plans that go beyond the guidance the Trump administration sent to states earlier on Wednesday. Biden said his plans, unlike Trump's, will include a detailed timeline for when people will be able to get vaccination and a breakdown of which populations will be at the front of the line, based on advice from medical experts and ethicists.

In response to a question following the speech, Biden also clarified his position on a national mask mandate, reiterating that he would try to pressure and convince holdout governors to implement statewide rules but adding that his legal team is currently exploring options for a federal executive order if that proves unsuccessful.

"I would make the case why it's necessary," he said. "I would have the scientists lay out in detail why, and I would go to every governor -- Republican and Democratic governors -- and I would say, 'We have to have this national mandate. We must do it.'"

He said that if he finds he has the legal authority to implement a national mask mandate over the objection of governors, he would do so " based upon the degree to which there is a crisis in those states."