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President Trump’s litany of recent attacks on Joe Biden has created an opportunity for the former vice president to argue that he is best equipped to beat Mr. Trump.

Mr. Trump has come under fire over reports last week that he repeatedly asked the president of Ukraine to investigate Mr. Biden and his son, who had business dealings in Ukraine while Mr. Biden was vice president.

Mr. Trump has denied threatening to withhold military aid for Ukraine if Kiev didn’t pursue an investigation of Mr. Biden, but the president has doubled down on his public criticism of the former vice president, arguing, without evidence, that Mr. Biden was involved in a scheme in Ukraine to protect his son.

Reports of Mr. Trump’s interactions with Ukraine have drawn widespread condemnation from Democrats, who are calling on the administration to give Congress access to a whistleblower complaint that is partly related to the Ukraine call. Some Republicans are also looking for more information about Mr. Trump’s call with Ukranian President Volodymyr Zelensky and the whistleblower complaint.