WASHINGTON, D.C. (NEXSTAR) — Just minutes before he was supposed to testify in front of Congress Tuesday, the State Department said Ambassador Gordon Sondland would not be allowed to speak about his knowledge of a call between President Donald Trump and the Ukranian Prime Minister.

The call is at the center of a whistleblower complaint that alleges Trump pressured Ukraine to investigate political rival Joe Biden in exchange for military aid.

House Democrats who’d hoped to learn more from Sondland, the U.S. ambassador to the European Union, were left frustrated and angry.

“The failure to produce this witness, we consider yet additional strong evidence of obstruction,” said Rep. Adam Schiff (D-Ca.), chairman of the House intelligence committee.

Yet House Republicans were quick to defend President Donald Trump and the State Department, which blocked Sondland’s testimony.

“It’s base on the unfair and partisan process that Mr. Schiff has been running,” said Rep. Jim Jordan (R-Oh.).

Some of Sondland’s text messages were released last week by Democrats hoping to strengthen their case that Trump pressured Ukraine to investigate political rival Joe Biden ahead of next year’s presidential election.

The American people have the right to know if the president is acting in their interests and not in his own narrow, personal political interest,” Schiff said.

“This is a scam by the Democrats to try to win an election they’re not going to win in 2020,” Trump said.

But some experts said the White House’s strategy of blocking key testimony might end up making Democrat’s case for impeachment easier.

“Where it is likely to end up is in an article of impeachment based on the resistance to providing evidence,” said Gene Healy of the Cato Institute.

Healy said House Democrats may decide to skip lengthy court battles over subpoenas and instead rely on the White House’s own resistance.

“Congress doesn’t have any really powerful tools at its disposal to quickly force the executive branch to hand over information,” Healy said.

Still, House Democrats said they plan to subpoena Sondland’s testimony, making it harder, but not impossible, for the White House to stop him.