Four of the country's 50 states have already voted, but March 3 is the biggest day of the entire presidential primary process, with tens of millions of Americans eligible to cast ballots.
The win was seen as crucial to Biden's hopes of challenging Bernie Sanders, for the spot on the Democratic ticket in November against Republican Donald Trump.
Polling released by Bloomberg's campaign on Tuesday showed voters in 42 of the most vulnerable House Democrats' districts preferring Trump to Sanders.
US prosecutors have charged the 48-year-old Australian computer hacker with espionage over WikiLeaks' hacking of thousands of confidential government documents.
Biden said Sanders' supporters insulted and made online threats against leaders of an influential union that declined to endorse any of the 8 candidates still in the Democratic race.
The budget would raise military spending by 0.3% to $740.5 billion for the fiscal year 2021, starting October 1 and propose higher outlays for defense and veterans.
Donald Trumps aides and allies are pressing for him to avoid confrontation with Democratic critics at the State of the Union speech, due to the ongoing impeachment trial against him.
The Iowa State Party said it was doing 'quality checks' on the results, and a state party source said a new mobile app designed to report the vote had failed, slowing the results.
Joe Crowley, the fourth most powerful Democrat in the House of Representatives, had been considered a potential replacement for Nancy Pelosi, democrats reclaim the majority in November's midterm elections.