Ukraine's Zelenskyy tells Congress: 'We are united ... the entire free world'

Zelenskyy’s trip to Capitol Hill will be similar to State of the Union preparations because of the high-level nature of the address. Security officials at the most senior levels of government are "very" concerned about the prospect of something happening tonight, domestically or abroad, one source told ABC News.

According to an email sent to staff at the Capitol and obtained by ABC News, security measures will be "significant." The email says only staff and members will be allowed in the House wing past a certain time.

Another official said, “We are very cognizant that Russia has assets in this country and might try to do something. We know what is at stake.”

The source said the U.S. is aware that early on in the conflict the Russians apparently plotted to kill Zelenskyy. The official expressed concern that news of Zelensky’s visit broke so early about him coming to the United States. It would have been much better, he said, if our adversaries had less time, not more time, to think about doing something and to move assets and operatives around.

-ABC News Chief Justice Correspondent Pierre Thomas and Luke Barr

Zelenskyy's trip to Washington is being treated as a mini-state visit -- a visit with extraordinary security implications, according to multiple sources. Hundreds of law enforcement and intelligence officials have been activated for this visit with the U.S. Secret Service tasked as the lead agency.

"From the moment he lands and walks down those stairs of his plane, he will have a Secret Service security detail," one official tells ABC News. "He will have that detail until he gets on the plane to leave."

Secret Service is also consulting with the Capitol Police, CIA, FBI and other intelligence agencies about the security environment. One source told ABC News every Capitol Police officer is on standby.

-ABC News' Chief Justice Correspondent Pierre Thomas and Luke Barr

Senate Majority Leader Chuck Schumer, D-N.Y., compared Zelenskyy to Winston Churchill during floor remarks on Wednesday.

"Where Winston Churchill stood generations ago, so, too, President Zelenskyy stands not just as a president but also as an ambassador to freedom itself," Schumer said.

Churchill, prime minister of the United Kingdom during World War II, addressed U.S. lawmakers in a speech the day after Christmas in 1941 -- just weeks after the Pearl Harbor attack.

"Sure I am, that this day, now, we are the masters of our fate," Churchill told Congress. "That the task which has been set us is not above our strength. That its pangs and toils are not beyond our endurance. As long as we have faith in our cause, and an unconquerable willpower, salvation will not be denied us."

Schumer said he will "happily join" Congress in welcoming Zelenskyy, and urged Republicans to attend the joint meeting.

"It is a high honor to welcome a foreign head of state to Congress, but it is nearly unheard of to hear from a leader who is fighting for his life fighting for his country's survival and fighting to preserve the very idea of democracy," Schumer said.

At the White House, Biden will have an "in depth strategic discussion" with Zelenskyy today about how to move forward on the battlefield, capabilities and training for Ukraine, sanctions and export controls, and the economic and energy sector, a senior administration official told reporters.

From the initial discussion of a possible visit to its final confirmation, this all came together in one week.

Biden and Zelesnkyy discussed this visit when they spoke over the phone on Dec. 11 and the White House extended a formal invitation on Dec. 14 for the Ukrainian president to visit Washington on Dec. 21. Zelenskyy's office formally accepted the invite on Friday, Dec. 16 and the visit was confirmed on Sunday.

"We wanted to host him for a program here at the White House that would involve an extended sit down with President Biden, a meeting with key members of President Biden's national security team and Cabinet, a opportunity to address the press and then an opportunity to go up to Capitol Hill to do a Joint Session of Congress."

The official stressed that ultimately this was Zelenskyy's decision to make to visit Washington, dismissing a question on whether Russia could take drastic actions with him out of the country, saying they've already been doing that "on a near daily basis."

–ABC News' Justin Gomez