WASHINGTON (AP) — 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.

A quick summary of the latest news:

WHAT YOU NEED TO KNOW

President Donald Trump is confronting the limits of his main impeachment defense. Trump and his aides have largely ignored the details of the Ukraine allegations against him. Instead, they're loudly objecting to the House Democrats' investigation process. The strategy is a less-than-compelling counter to the mounting threat to Trump's presidency.

Most Republicans are still standing by Trump with a range of reactions from cautious, poorly coordinated responses critical of Democrats' handling of the impeachment probe to storming a Capitol Hill hearing to disrupt testimony a day after Tuesday's closed-door testimony by acting ambassador William Taylor.

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WHAT'S NEXT

Phil Reeker, the acting Assistant Secretary of State for Europe, is scheduled to provide testimony to House investigators on Saturday.

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NUMBERS THAT MATTER

If President Donald Trump is impeached by the House, the Senate becomes the trial court that will hear the charges against him. It takes two-thirds of the chamber, 67 votes, to convict a president and remove him from office.

Republicans hold a narrow majority of 53 members. It would take 20 GOP defections to get a conviction, assuming all 45 Democrats and two independents vote to convict. Two presidents have been impeached — Andrew Johnson in 1868 and Bill Clinton in 1998 — and both were acquitted by the Senate. President Richard Nixon resigned in 1974 before articles of impeachment reached the House floor for a vote.

Senators will have to weigh not only the charges against the president, but how their constituents will hold them accountable for their votes.

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WORTH WATCHING

President Trump's GOP allies are trying to draw attention to the process behind the Democratic-run impeachment inquiry rather than the substance of it, arguing that the closed-door testimony keeps Republican members and the public from seeing what evidence is being gathered against the president:

https://youtu.be/X-7DGaZE2GE