Trump campaign manager Brad Parscale also echoed the White House message. Democrats, "should just get on with it so we can have a fair trial in the Senate and expose The Swamp for what it is. Speaker Pelosi, Chairman Schiff, and Hunter Biden should testify, and then we can get back to the business of our country," Parscale said in a statement.

Earlier Thursday, the president tweeted that if Democrats planned to impeach him, they should "do it now ... so that our Country can get back to business."

"The president's actions have seriously violated the Constitution," Pelosi said at a news conference on Capitol Hill , adding that Trump, "leaves us no choice but to act."

WASHINGTON — Moments after House Speaker Nancy Pelosi gave the go-ahead Thursday to move forward on impeachment, White House press secretary Stephanie Grisham said "we look forward to a fair trial in the Senate."

Pelosi's comments confirm what was widely expected: that Democrats in the House will vote on whether to impeach the president.

"Sadly, but with confidence and humility, with allegiance to our founders, and our heart full of love for America, today I am asking our chairman to proceed with the articles of impeachment," she said.

Trump posted two statements on impeachment Thursday morning, one before Pelosi's announcement, and one after.

In his earlier statement, Trump struck an almost-upbeat tone, writing, "Therefore I say, if you are going to impeach me, do it now, fast, so we can have a fair trial in the Senate, and so that our Country can get back to business. We will have Schiff, the Bidens, Pelosi and many more testify, and will reveal, for the first time, how corrupt our system really is. I was elected to 'Clean the Swamp,' and that's what I am doing!"

Shortly after Pelosi's statement, however, Trump tweeted again. "The Do Nothing, Radical Left Democrats have just announced that they are going to seek to Impeach me over NOTHING," he wrote. "They already gave up on the ridiculous Mueller 'stuff,' so now they hang their hats on two totally appropriate (perfect) phone calls with the Ukrainian President.

"This will mean that the beyond important and seldom used act of Impeachment will be used routinely to attack future Presidents," Trump continued. "That is not what our Founders had in mind. The good thing is that the Republicans have NEVER been more united. We will win!"

House Democrats are weighing whether Trump should be removed from office for holding up nearly $400 million in congressionally authorized military assistance to Ukraine as it battled pro-Russian separatists in return for allegedly pressuring Ukraine's president to launch an investigation of Trump's political rivals.

The Judiciary Committee, led by Rep. Jerrold Nadler, D-N.Y., will be responsible for drafting specific articles of impeachment. On Wednesday, Nadler's committee held a public hearing with four legal scholars who discussed whether Trump's efforts toward Ukraine met the constitutional bar for impeachment.

That hearing followed weeks of private and public testimony in the House Intelligence Committee, led by Chairman Adam Schiff, D-Calif.

-- CNBC's Kevin Breuninger contributed to this report.