The White House has officially refused to co-operate with the impeachment inquiry against President Donald Trump.
A letter sent to Democratic leaders rejected it as “baseless” and “constitutionally invalid”.
“In order to fulfil his duties to the American people… President Trump and his Administration cannot participate in your partisan and unconstitutional inquiry under these circumstances.”
In response, Ms Pelosi called the letter “manifestly wrong” and accused Mr Trump of trying to “normalise lawlessness”.
The letter, written by Trump’s lawyer Pat Cipollone, accuses Democrats of trying to change the 2016 election result, and attacks the investigation as “constitutionally invalid and a violation of due process”.
The White House letter comes hours after the Trump administration blocked the US ambassador to the European Union from appearing before a congressional impeachment investigation.
Three Democratic-led House committees are investigating Mr Trump.
The inquiry is trying to find out if the president held back aid to Ukraine to push it to investigate Joe Biden, who is running for the Democratic presidential nomination.
In a July 25 phone call, Trump allegedly made military aid contingent on Ukrainian President Volodymyr Zelensky launching a probe into Joe Biden, a former U.S. vice president, and his son, Hunter, a businessman, over unsubstantiated corruption allegations.
The elder Biden is a leading candidate in the race to win the Democratic nomination and challenge Trump in 2020. Trump, a Republican, has repeatedly denied any wrongdoing and accused Democrats of time-wasting.