U.S. Vice President Mike Pence and President Donald Trump’s personal lawyer Rudy Giuliani said on Tuesday they will not cooperate with a U.S. House of Representatives impeachment inquiry, prompting a leading Democrat to say that would strengthen the case against the president.

The Pentagon also said it would not comply with lawmakers’ request for documents related to Trump’s effort to pressure Ukraine to investigate a political rival, further illustrating Trump’s determination to stonewall the Democratic-led impeachment effort, which threatens to consume his presidency.

“The evidence of obstruction of Congress continues to build,” Democratic Representative Adam Schiff, one of the leaders of the impeachment effort, said at a news conference.

Other U.S. government officials have not been as reluctant to cooperate.