Tonight, "60 Minutes" has obtained a letter that indicates the government whistleblower who set off the impeachment inquiry of President Trump is under federal protection, because he or she fears for their safety. These rapidly developing events began Tuesday when Speaker of the House Nancy Pelosi ordered the investigation based on a phone call between Mr. Trump and the president of Ukraine. President Volodymyr Zelensky asked Mr. Trump for missiles, Mr. Trump asks Zelensky for "a favor" to investigate Mr. Trump's Democratic rivals.

The letter from the whistleblower's lawyers to the acting director of national intelligence

Democrats say this is the type of collusion that was the focus of the Mueller investigation. And it appears Washington will be immobilized by this, 13 months before election day. President Trump says he is the victim of a Democratic smear, crooked media and treasonous spies. Tonight we will hear from the man in charge of the investigation, the president's lead defender in Congress and Speaker Pelosi who, for months, resisted impeachment.

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Nancy Pelosi: We could not ignore what the president did. He gave us no choice. So it wasn't any change of mind. I always said we will follow the facts where they take us. And when we see them, we will be ready. And we are ready.

Speaker of the House Nancy Pelosi

Early last week, details of the president's phone call filtered out in the press. As some at the Capitol called for impeachment, Mr. Trump phoned Speaker Pelosi to reassure her about the call with Zelensky.

Scott Pelley: He told you about the phone call?

Nancy Pelosi: He told me it was perfect. There was nothing in the call. But I know what was in the call. I mean, it was in the public domain. He didn't even know that it was wrong. You know, he was saying, "It was perfect. There was nothing wrong." Well, no, it is wrong. It is wrong for a president to say that he wants you-- another head of state-- to create something negative about his possible political opponent to his own advantage, at the expense of our national security, his oath of office to the Constitution and the integrity of our elections.

The facts are these: on July 25, Mr. Trump was celebrating his new defense secretary in public, but two hours before, he spoke with Ukrainian President Volodymyr Zelensky. Zelensky was interested in Javelin anti-tank missiles to defend himself from Russian-backed rebels.

The White House record of President Trump's call with Ukrainian President Volodymyr Zelensky

This is the official White House record of the call. Zelensky: "We are almost ready to buy more Javelins from the United States for defense purposes." Mr. Trump replied, "I would like you to do us a favor though…" Mr. Trump then asked Zelensky to investigate a theory about a supposed Democratic National Committee computer server. "The server," Mr. Trump said, "they say Ukraine has it…" He offered the assistance of the U.S. government. "I would like to have the attorney general call you or your people and I would like you to get to the bottom of it."

The call came to light after a U.S. intelligence official heard about it and filed an official government whistleblower complaint. The unnamed intelligence officer writes, "…I have received information from multiple U.S. government officials that the president of the United States is using the power of his office to solicit interference from a foreign country in the 2020 U.S. election."

The whistleblower complaint

Adam Schiff: Here is our ally, here is Ukraine, struggling militarily to fight off Russian aggression struggling to be a democracy and what is the president telling Ukraine through his words and his deeds.

Democrat Adam Schiff is chairman of the House Intelligence Committee, in charge of the investigation.

Scott Pelley: Where does your committee take this from here? What's the procedure?

Adam Schiff: Well, we have a pretty good roadmap-- thanks to the courage of this whistleblower. The complaint sets out any number of witnesses, any number of documents that we need to seek.

Scott Pelley: Do you expect the testimony of the whistleblower?

Adam Schiff: Absolutely.

Scott Pelley: Your committee already has an agreement with the whistleblower that he will testify?

Adam Schiff: We have an agreement that he or she will testify, yes.

Adam Schiff, chairman of the House Intelligence Committee

Schiff told us that part of his focus is the president's personal lawyer, Rudy Giuliani, who has been encouraging Ukraine to investigate Hillary Clinton and Joe Biden. Giuliani denies any wrongdoing.

Scott Pelley: Will you call Rudy Giuliani?

Adam Schiff: We're gonna need evidence from Rudy Giuliani. And it's our intention as soon as first thing next week to subpoena him for documents. And there may very well come a time where we wanna hear from him directly.

Giuliani asked Ukraine to investigate Hunter Biden, the former vice president's son who was on the board of a Ukrainian company that was under investigation there. No evidence surfaced that Hunter Biden did anything illegal. But, during the Obama administration, Vice President Biden pressured Ukraine to fire its prosecutor general, a man western governments considered to be corrupt. This left the Bidens with, at least, the appearance of a conflict of interest. In his call with President Zelensky, Mr. Trump said, "the other thing, there's a lot of talk about Biden's son, that Biden stopped the prosecution and a lot of people want to find out about that so whatever you can do with the attorney general would be great." U.S. Attorney General William Barr has denied being involved.

Kevin McCarthy: When I read the transcript, I see two leaders having admiration, not intimidation.

Kevin McCarthy is the leader of House Republicans and heading the effort in the House against impeachment.

Scott Pelley: What do you make of this exchange? President Zelensky says, "We are almost ready to buy more Javelins from the United States for defense purposes." And President Trump replies, "I would like you to do us a favor though."

Kevin McCarthy: You just added another word.

Scott Pelley: No, it's in the transcript.

Kevin McCarthy: He said- "I'd like you to do a favor though"?

Scott Pelley: Yes, it's in the White House transcript.

Kevin McCarthy: When I read the transcript, President Zelensky brings up a Javelin is a protection for anti-tank, something that President Obama would not sell that President Trump did to protect the Ukraine.

Scott Pelley: How do you expect the president's defense to roll out going forward?

Kevin McCarthy: The defense of what?

Scott Pelley: Well, there's an impeachment inquiry.

Kevin McCarthy: Yeah. There's an impeach inquiry going forward. It probably never would move forward, had the speaker waited 48 hours to have the transcript. We vote on important things every day. But there are certain votes that are different than others. Sending men and women off to war is the most difficult vote any member of Congress would ever make.

Scott Pelley: I'll ask you again, how does the defense of the president, in your view, roll out from here?

Kevin McCarthy: Why would we move forward with impeachment? There's not something that you have to defend here.

Scott Pelley: You say the president has done nothing wrong. I take that to mean that you find it appropriate that the president asked Mr. Zelensky for an investigation of his Democratic rivals.

Kevin McCarthy: The question before the House of Representatives is to impeach the president based upon a phone call that the speaker never even heard…

Scott Pelley: Mr. Leader, with great respect to you, and I apologize for interrupting, but these are the White House talking points that were emailed to the Congress earlier this week.

Kevin McCarthy: Well, I'll be very c--

Scott Pelley: And I am asking you was it appropriate for the president to ask for investigations of his Democratic rivals with another foreign leader?

Kevin McCarthy: I've never seen one talking point from a White House. I'm talking to you based upon the most important facts we have. The whistleblower wasn't on the call. The IG, inspector general, didn't read the call. But you and I have all the information we need. The president did nothing in this phone call that's impeachable.

House Minority Leader Kevin McCarthy

According to the whistleblower complaint, "White House officials understood the gravity of what had transpired in the call." The whistleblower says he was told the record of the call was removed from the usual computer system to "…a separate electronic system that is otherwise used to store and handle classified information of an especially sensitive nature."

Wednesday, the day the record of the call was revealed, Mr. Trump met in New York, face-to-face, with the man on the other end of the phone, Ukrainian President Zelensky. Zelensky was asked if he felt pressured.

President Zelensky: It was normal we spoke about many things and I, so I think, and you read it that nobody push it—pushed me.

President Trump: In other words no pressure. You know what? There was no pressure and, by the way, you know there was no pressure all you have to do is see it—what went on in the call.

But it's not just the call. Investigators believe White House pressure began months before. Vice President Pence cancelled plans to attend Zelensky's inauguration. Then, President Trump suspended nearly $400 million in aid to Ukraine that Congress had authorized. Against this backdrop, the president asked for "the favor."

The impeachment inquiry began while Mr. Trump was meeting world leaders in New York.

President Trump: It's all a hoax folks. It's all a big hoax. And the witch hunt continues but they're getting hit hard in this witch hunt because when they look at the information, it's a joke. Impeachment? For that?

When Mr. Trump visited America's U.N. staff, it appeared he threatened whoever revealed the call.

President Trump: "I want to know who's the person who gave the whistleblower – who's the person who gave the whistleblower the information? Because that's close to a spy. You know what we used to do in the old days when we were smart? Right? These spies and treason, we used to handle it a little differently than we do now."

The president's remarks prompted the whistleblower's lawyers, this weekend, to send a letter to the acting director of national intelligence. They thank the director for activating "appropriate resources" to ensure their client's safety. They write that "certain individuals" are offering a "$50,000 bounty" for their client's identity.

Scott Pelley: The president has suggested that the people behind this are spies and perhaps guilty of treason.

Adam Schiff: It's hard to describe how dangerous and loathsome that invitation to violence is.

Adam Schiff hopes to begin hearing witnesses this week.

Scott Pelley: Your Republican colleagues say, "We just went through this. That the Mueller report was inconclusive, you drug the country through this for two years and now we're going to do this again?

Adam Schiff: After the last two years that we've been through, the president well understood that it was illegal to seek foreign assistance in a campaign. And immediately after Mueller testified, that is exactly what he was back at doing again.

Special counsel Robert Mueller testified about his investigation into Russian interference in the 2016 election on July 24. Mr. Trump's call with Zelensky was the next morning.

Scott Pelley: Will any Republican vote in favor of impeachment, in your view?

Kevin McCarthy: Having seen the transcripts, having listened to my conference, I haven't heard one member from any element inside there tell me this rises to impeachment.

Since our interview, one Republican representative, Mark Amodei, announced support for the impeachment inquiry. Amodei was chair of the Trump 2016 Election Campaign in Nevada.

Scott Pelley: Your Republican colleagues say, "Well, the call is the call but there's nothing here that rises to the level of high crimes and misdemeanors."

Nancy Pelosi: Well, they're wrong. And it remains to be seen because it's not just what happens in the call. It's part of the sequencing of events as well. You withdraw a couple hundred million dollars-worth of assistance to a country. And then a couple days later, say, "By the way, can you help me with my campaign," in other words. There's a sequencing there.

Scott Pelley: What is your message to the White House in terms of cooperation?

Nancy Pelosi: To the White House? Speak the truth. Honor your oath of office to the Constitution of the United States. Speak the truth. And let us work together to have this be a unifying experience, not a dividing one for our country. Don't make this any worse than it already is.

The Trump administration appointed a veteran U.S. diplomat as special envoy to Ukraine. Kurt Volker put Mr. Trump's lawyer, Rudy Giuliani in touch with Ukrainian officials. Volker abruptly resigned on Friday. He's expected to testify to the committee.

Produced by Nicole Young, Ashley Velie, Henry Schuster and Maria Gavrilovic. Associate producers, Rachael Morehouse, Dina Zingaro, Alex Ortiz, Katie Kerbstat