The revelations offered voters a preview of what is likely to be an extraordinary general election contest if Mr. Biden were to win the nomination, one in which attacks by the president and his team could boomerang, transforming Mr. Biden into a sympathetic figure under ugly attack with foreign help.

It could just as easily mark a defining moment for Mr. Biden, a 76-year-old politician first elected to the Senate in 1972 and long accustomed to playing by the more genteel political rules of a different era…

Even if Mr. Biden’s primary competitors don’t take direct aim, the perception of the Biden family leveraging its connections — even if little more than a conspiracy theory — cuts a stark contrast with his two leading rivals, Senators Elizabeth Warren and Bernie Sanders, who have centered their candidacies around a fierce populist message of rooting out corruption in Washington.

It’s a message that worked in 2016 for Mr. Trump, who cast Hillary Clinton as the avatar of establishment self-dealing, a past-her-prime creature of Washington unable to adjust to the times and produce real change.