Rick Gates, Paul Manafort’s right-hand man for years, began testifying against his former boss on Monday in federal court in Alexandria, Va. He is considered the most important witness in Mr. Manafort’s trial on tax and bank fraud charges.

The outcome of the closely watched trial, now in its second week, could hinge on whether the jury believes the testimony from Mr. Gates, 46, who has pleaded guilty to conspiracy to commit fraud and lying to federal authorities and faces a prison term of up to six years.

Mr. Gates’s testimony also underscores the trial’s stakes for the White House.

Mr. Manafort was Mr. Trump’s campaign chairman, but resigned from the campaign in August 2016 after just five months. Mr. Gates, the deputy chairman, remained on the campaign as a liaison to the Republican National Committee through the election. Mr. Gates was named deputy chairman of Mr. Trump’s inaugural campaign, raising huge sums for the event.

The trial is the first stemming from charges brought by the special counsel, Robert S. Mueller III, who is investigating Russia’s attempts to influence the 2016 election.