WASHINGTON — The top American diplomat in Ukraine identified Rudolph W. Giuliani, President Trump’s personal lawyer, as the instigator behind the drive to get Ukraine’s president to announce investigations into Mr. Trump’s political rivals, telling impeachment investigators last month that Mr. Giuliani was acting on behalf of the president.

House Democrats on Wednesday released a transcript of the private testimony by the diplomat, William B. Taylor Jr., as they named him as the first of several witnesses who will testify publicly next week in a slate of impeachment hearings. They will begin laying out a case that Mr. Trump abused his office to secure political favors from Ukraine.

Lawmakers plan to question Mr. Taylor and George P. Kent, a senior American diplomat who oversees policy in the region, during a televised joint session on Wednesday. Then on Friday, they will hear from Marie L. Yovanovitch , the former American ambassador to Ukraine, about her abrupt recall to Washington this spring amid a campaign to smear her as disloyal.

The announcement, after six weeks of fact-finding that largely took place in the Intelligence Committee’s secure chambers, was a sign that Democrats now feel they have assembled a strong enough record to present voters. The hearings will also almost certainly usher in a new, more intense round of partisan warfare as Republicans try to blunt what they see as an existential threat to Mr. Trump’s presidency.