EU ambassador to be deposed as impeachment inquiry rolls on

For only the fourth time in U.S. history, the House of Representatives has started a presidential impeachment inquiry. House committees are trying to determine if President Donald Trump violated his oath of office by asking a foreign country to investigate a political opponent.

Here's a quick summary of the latest news.

WHAT YOU NEED TO KNOW

— House Democrats issued subpoenas Monday to Defense Secretary Mark Esper and acting White House budget director Russell Vought.

— House Speaker Nancy Pelosi's office released an open letter signed by 90 former national security officials who served in both Democratic and Republican administrations, voicing support for the whistleblower who raised concerns about Trump's efforts to get Ukraine to investigate political foe Joe Biden.

— Trump said the impeachment inquiry is a "scam" that is driving his poll numbers higher, but admitted it "makes it harder to do my job."

— Sen. Rob Portman of Ohio became the fourth Republican senator to break ranks by criticizing Trump for seeking help from foreign governments to investigate a political rival. However, Portman said he does not "view it as an impeachable offense" and believes the House "rushed to impeachment assuming certain things."

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WHAT'S NEXT

Staff and lawmakers from the House Intelligence Committee, the House Oversight and Reform Committee and the House Foreign Affairs Committee will continue to hear from witnesses throughout the week. Gordon Sondland, the U.S. ambassador to the European Union who has become a key figure in the probe, will be deposed Tuesday. Marie Yovanovitch, the former U.S. ambassador to Ukraine who was recalled from the post early, will be speaking Friday.

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NUMBERS THAT MATTER

A solid majority of House Democrats — 226 out of 235 voting members — back the impeachment inquiry, according to an AP tally. They are joined by Republican-turned-independent Justin Amash of Michigan. Only five Democrats oppose the probe or are undecided about it; four have not confirmed a position to the AP.

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WATCH THIS

Energy Secretary Rick Perry, who has been drawn into the House impeachment probe as the person Trump said teed up the July 25 call with Ukrainian President Volodymyr Zelenskiy, said in Lithuania Monday that he asked Trump "multiple times" to speak with Zelenskiy.

Spokeswoman Shaylyn Hynes said Perry wanted Trump to speak on energy matters with the Ukrainian leader as part of U.S. efforts to boost Western energy ties to Eastern Europe. A rough transcript of the call shows Trump pressing Zelenskiy to investigate Biden and his son.

Perry also dismissed rumors of his impending departure from the Trump administration, saying reports of his exit are premature. "Not today, not tomorrow, not next month," he said.