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Impeachment process: How will this work

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(CNN) -- Calls to impeach President Donald Trump began among some Democrats almost the day he was inaugurated. They ballooned after former special counsel Robert Mueller's report to Congress about interactions between the Trump campaign and Russia and, separately, Trump's efforts to squash the investigation, potentially obstructing justice.

But now that Trump has admitted to trying to influence Ukrainian President Volodymyr Zelensky to investigate Joe Biden at the same time he had frozen US military aid to Ukraine, the calls among Democrats have reached a tipping point.

Here's what needs to happen for Democrats in the House to impeach Trump, which seems increasingly likely, and for the Senate to remove him from office, which still seems very unlikely. Much of the information here is taken from a report by the Congressional Research Service and other sources.

Initiating impeachment proceedings

The two most recent presidential impeachments began with a vote in the full House. It appears that Pelosi has chosen to instruct the six House committees already investigating Trump to proceed "under that umbrella of impeachment inquiry."

Rep. Jerry Nadler, the current chair of the House Judiciary Committee, has argued that his current oversight of the Trump administration equaled the beginnings of an impeachment inquiry.

Formalizing it could help the committee in its effort to get Trump administration officials to cooperate. Instead, they have at times stonewalled, claimed executive privilege, and ignored subpoenas.

Investigating impeachable offenses

Impeachable offenses, according to the Constitution, include "Treason, Bribery, or other high Crimes and Misdemeanors." What exactly constitutes those crimes, particularly High Crimes and Misdemeanors, has been the subject of vigorous debate throughout US history.

During an impeachment inquiry, a House committee, usually the Judiciary Committee or its subcommittee, conducts an investigation to see if a federal official's conduct warrants impeachment.

This will be a contentious process.

Clinton sent lawyers and legal experts to argue his case and defend him before the House Judiciary Committee.

Nixon defied the committee's request for tapes of his Oval Office conversations. Ultimately, the Supreme Court ruled he had to turn them over to a special prosecutor and that process unfolded in tandem with the impeachment process.

House votes

After its inquiry, the committee conducting the impeachment inquiry writes articles of impeachment and votes on whether to refer them to the full House of Representatives. After that vote, the articles, if approved, are given special status on the House floor and it requires a simple majority of voting lawmakers to approve them.

Here are the articles of impeachment offered against President Bill Clinton.

Here are the articles of impeachment prepared for Richard Nixon.

Currently, Democrats hold a 235 to 198 majority over Republicans. Michigan Rep. Justin Amash, who left the Republican Party in July after announcing his support for impeachment proceedings, is the chamber's one independent and there is one vacancy. Assuming Amash votes with Democrats, they could afford to lose 18 Democrats and still achieve a majority.

At this point, Trump would face a choice shared by only three other presidents: be impeached and fight for your office in the Senate or resign. Both Clinton in 1999 and President Andrew Johnson in 1868 fought in the House and ultimately survived a Senate trial. Nixon, after learning that Republicans would not support him during impeachment, resigned before the House could vote to impeach him.

There were four articles of impeachment against Clinton, but only two were approved, on mostly party-line votes with a few defections.

A Senate trial

After the House votes to impeach a President, the Constitution calls for a trial in the US Senate. There is some question as to whether Republicans in the Senate would even bother since they likely have the votes to easily dismiss the charges.

Senate Majority Leader Mitch McConnell simply decided not hold a vote on Merrick Garland's nomination by President Barack Obama to sit on the Supreme Court. There's been plenty of speculation about whether McConnell would simply decide not to hold an impeachment trial.

If there were a trial, senators become jurors and the chief justice of the US Supreme Court presides -- that's John Roberts. The House votes on managers who bring the trial in the Senate. Although they might be helped by outside counsel, the House managers actually bring the trial and present evidence. The impeached official can offer a defense or have counsel do it.

While Democrats hold a majority in the House, Republicans control the Senate with a 53-45 majority. Two independents usually side with Democrats. But it takes a supermajority to remove a President from office. That means 67 senators would have to agree that Trump should be removed from office.

With the current party split, 20 Republicans would have to turn on him. At the moment, exactly zero Republicans senators have said anything close to supporting the idea of impeachment.

How long does this take?

This process can take months. For Johnson, the entire process lasted 94 days, from first congressional action to Senate acquittal, lasted from February 22, 1868 to May 26, 1868.

For Richard Nixon, it lasted 184 days. The House approved the impeachment inquiry on February 6, 1974 and Nixon resigned.

For Clinton, it lasted 127 days. The House approved the impeachment inquiry on October 8, 1998, and the Senate acquitted him on February 12, 1999.

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