An associate of Rudolph W. Giuliani who was involved in a campaign to pressure Ukraine into aiding President Trump’s political prospects has broken ranks, opening a dialogue with congressional impeachment investigators and accusing the president of falsely denying their relationship.

The associate, Lev Parnas, had previously resisted speaking with investigators for the Democrat-led impeachment proceedings, which are examining the president’s pressure attempts in Ukraine. A former lawyer for Mr. Trump was then representing Mr. Parnas.

But since then, Mr. Parnas has hired new lawyers who contacted the congressional investigators last week to notify them to “direct any future correspondence or communication to us,” according to a copy of the letter.

The lawyers also signaled on Monday that Mr. Parnas, who was arrested last month on campaign finance charges, is prepared to comply with a congressional subpoena for his documents and testimony.