The United States Democratic lawmakers have demanded documents from the White House as part of their impeachment inquiry into President Donald Trump.
The documents relate to a call between Trump and Ukrainian President Volodymyr Zelensky on 25 July.
In the call, Trump pushed Zelensky to investigate his leading Democratic political rival, Joe Biden.
The impeachment inquiry stems from the call, which was flagged up by a whistleblower in August.
The whistleblower alleged that Trump used a $400m military aid package to Ukraine, which had been suspended earlier in July, as leverage to persuade Mr Zelensky. The White House released the aid in September.
Trump has denied any wrongdoing, accusing his political opponents of a "witch hunt".
"We deeply regret that President Trump has put us - and the nation - in this position, but his actions have left us with no choice but to issue this subpoena," the Democrats wrote in a letter to the White House.