WASHINGTON — When a reporter asked former Vice President Joseph R. Biden Jr. on Saturday if he had spoken to his son Hunter about his work in Ukraine, Mr. Biden jabbed his finger repeatedly at the reporter and shot back: “Ask the right questions.”

A few hours later, when coverage of President Trump’s phone call to Ukraine’s president about Mr. Biden and his son had reached a fever pitch, the Biden campaign emailed the news media to declare — in bold underlined words — that any story would be “misleading” if it did not state upfront that Mr. Trump’s claims were unsubstantiated.

On Sunday night, the Biden campaign released a fund-raising appeal on Twitter asking, “Will media see through Trump’s sleazy playbook ? Or fall for it again?” And by Tuesday, the drumbeat of warnings to the press had grown so intense that even Lanny Davis , a longtime Democratic operative, weighed in, attacking “innuendo journalism.”

For the past three years, every Democratic campaign, politician and activist has operated in the shadow of Hillary Clinton’s 2016 loss, caught in a cycle of self-recrimination and soul-searching about how to defend themselves against Mr. Trump and his Republican allies in 2020. It’s a conversation that began in the hours after Mrs. Clinton’s unexpected loss, as aides reckoned with their inability to defend her from Mr. Trump’s conspiratorial accusations, and has continued ever since, with Mrs. Clinton herself dispensing advice in private conversations with Democratic primary candidates as they prepared to begin their campaigns.