A look at what’s happening with Donald Trump’s impeachment hearings

Democrats are planning a rapid start to their push for impeachment of U.S. President Donald Trump this week.

The president has remained defiant amid the process. On Monday, he sent a series of tweets taking aim at the controversy.

Trump intensified attacks on the lawmaker leading the House of Representatives’ impeachment inquiry, suggesting that congressman Adam Schiff should be arrested for “treason.”

He also circulated quotes from an Evangelical pastor who appeared on Fox News, and suggested that impeaching Trump would lead to a “Civil War-like fracture in this nation.”

Pastor Robert Jeffress claimed that Democrats “don’t care if they burn down and destroy the nation.”

Meanwhile, Democrats have instructed committees to move quickly — and not to lose momentum — after revelations that Trump asked Ukrainian President Volodymyr Zelenskiy to investigate his potential 2020 Democratic challenger, Joe Biden, and his family.

The action is beginning even though lawmakers left town Friday for a two-week recess.

Witnesses and hearings

The committee is scheduled to hold a closed-door hearing on Friday with the intelligence community’s inspector general, Michael Atkinson, who has concluded that the whistleblower complaint was of urgent concern and appeared credible.

House investigators will also take the first witness testimony from two people mentioned in the whistleblower’s complaint.

On Wednesday, three House committees — Intelligence, Foreign Affairs and Oversight — are due to get a deposition from former U.S. ambassador to Ukraine Marie Yovanovitch, who Trump labelled “bad news” during his call with Zelenskiy.

On Thursday, the committees are set to get a deposition from Kurt Volker, who resigned last week as Trump’s special representative for Ukraine after the whistleblower complaint named him as one of two U.S. diplomats who followed up with Ukrainian officials a day after Trump’s call to Zelenskiy.

WATCH: Trump focusing attacks on whistleblower that triggered impeachment inquiry

1:39 Trump focusing attacks on whistleblower that triggered impeachment inquiry

Schiff’s committee has been negotiating to interview the whistleblower who began the firestorm by reporting that Trump had urged the investigations on a July phone call with Zelenskiy. Schiff told ABC’s This Week that his panel had reached an agreement to hear from the whistleblower, who would testify “very soon.”

Lawmakers on the committee say also want to speak to White House aides who were present for the call and to Rudy Giuliani, the president’s personal lawyer, who urged the investigations. Giuliani told ABC News Sunday that he doesn’t plan on co-operating, but will testify if Trump wants him to. Schiff says he hasn’t decided whether he wants to hear from Giuliani.

Democrats say they hope to finish the investigation in a matter of weeks.

House Judiciary Committee will intervene

Once the committees have finished their own investigations, they will submit their findings to the House Judiciary Committee, which oversees the impeachment process.

The Judiciary Committee chairman, Jerrold Nadler, has said he wants resolution on impeachment by the end of the year.

How Republicans are reacting

Republicans have focused their ire about impeachment on the Democrats, criticizing the probes as a rerun of a two-year investigation into Russian interference in the 2016 election.

California Rep. Devin Nunes, the top Republican on the Intelligence Committee, said Democrats “don’t want answers, they want a public spectacle.”

Senate Republican Whip John Thune said last week that he thinks the Democrats are taking on a “risky strategy.”

“I know they’re under a lot of pressure to do it, but if you’re the leadership over there, you got to think long and hard about what the implications are if it looks like you’re overreaching,” he said.

WATCH: Adam Schiff says whistleblower to testify soon, must ensure security of person

1:51 Adam Schiff says whistleblower to testify soon, must ensure security of person

When does the Senate get involved?

If the House votes to approve charges against Trump, the Republican-led Senate would then hold a trial.

For Trump to be impeached, it would take a two-thirds vote in the Senate to convict him and remove him from office.

Some Senate Republicans have expressed concerns about Trump’s interactions with Ukraine, but it’s still fairly unlikely they would take the bold step of convicting a president.

Republican Majority Leader Mitch McConnell said Monday the Senate would have “no choice” but to take up impeachment proceedings if they have House approval.

“How long you’re on it is a whole different matter, but I would have no choice but to take it up, based on a Senate rule on impeachment,” McConnell told CNBC.

— With files from The Associated Press, Reuters