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.

Here's a quick summary of the latest news:

WHAT YOU NEED TO KNOW

—The shifting White House explanation for President Donald Trump's decision to withhold military aid from Ukraine drew alarm Friday from Republicans as the impeachment inquiry tested their alliance.

—Former Ohio Gov. John Kasich, who ran against President Donald Trump in the 2016 Republican primaries, says he supports impeaching the president.

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WHAT'S NEXT

William "Bill" Taylor, the diplomat who expressed unease about a hold on security assistance for Ukraine, is expected to testify Tuesday.

Taylor at one point sent a text reading: "As I said on the phone, I think it's crazy to withhold security assistance for help with a political campaign." The text prompted the U.S. ambassador to the European Union, Gordon Sondland, to reply: "Bill, I believe you are incorrect about President Trump's intentions. The president has been crystal clear no quid pro quo's of any kind. The President is trying to evaluate whether Ukraine is truly going to adopt the transparency and reforms that President Zelensky promised during his campaign.

"I suggest," he added, "we stop the back and forth by text."

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WORTH WATCHING

An Associated Press-produced animation covers the basics of the impeachment process in less than two minutes: https://youtu.be/TSuLV_kDzeo .

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IN CASE YOU MISSED IT: Thursday's news conference by acting White House chief of staff Mick Mulvaney confirmed that the decision to hold up military aid to Ukraine was linked to a demand that Kyiv investigate the Democratic National Committee and the 2016 U.S. presidential campaign. Mulvaney later clarified his remarks, and Trump has stood by him. Video of Mulvaney's comments: https://youtu.be/iQFAh_MU69E .