WASHINGTON — President Donald Trump and his administration fumed Wednesday as the House prepared to vote to impeach him, and they prepared in turn for "war" over the move. Aides and allies said the anger stems from Trump's fear that impeachment will stain his legacy.

"SUCH ATROCIOUS LIES BY THE RADICAL LEFT, DO NOTHING DEMOCRATS. THIS IS AN ASSAULT ON AMERICA, AND AN ASSAULT ON THE REPUBLICAN PARTY!!!!" Trump wrote, shortly after the White House said in a statement that he was "working all day" but might "catch some of the proceedings between meetings.”

“Can you believe that I will be impeached today by the Radical Left, Do Nothing Democrats, AND I DID NOTHING WRONG! A terrible Thing. Read the Transcripts. This should never happen to another President again. Say a PRAYER!” Trump wrote in another, one of more than 30 tweets and retweets from his account by early afternoon on Wednesday.

The president spent most of the morning in the residence, according to a White House official. Upstairs in the White House, the televisions were tuned to cable news, which were all carrying the impeachment proceedings live.

“We are all mad,” a White House official told NBC News, describing the president's reaction as one of "disbelief" that the process had reached this point, and his team as being “angry this is happening.” But officials were quick to add that the president is ready for the fight ahead, describing the White House as battle-tested at this point.

LIVE BLOG: Follow the latest as the House prepares for Trump impeachment vote

The White House is readying for the fight to move into the Senate, too: White House counsel Pat Cipollone would likely still lead the effort, but the White House is also considering adding other voices to the fray.

Four sources familiar with the administration's thinking told NBC News that several high-profile GOP lawmakers are being considered to be part of the team that would defend the president in a Senate trial: Republican Reps. Jim Jordan of Ohio, Mark Meadows of North Carolina, John Ratcliffe of Texas, and Mike Johnson of Louisiana. One official said the situation is "fluid," with no decisions having been made.

While the Senate isn't expected to call formal witnesses, one source said the White House is considering trying to have House Republicans such as Rep. Doug Collins, R-Ga., serve as witnesses in some capacity to speak about the closed-door testimony and documents they've considered.

Trump made his feelings on impeachment clear in Tuesday’s six-page letter to House Speaker Nancy Pelosi, accusing Democrats of a “coup” — a document officials said he was likely to draw from in appearances later in the day.

Despite the president’s statement Tuesday that he did not plan to watch the impeachment debate, a senior administration official acknowledged as Wednesday began that Trump was likely to keep tabs on the proceedings.

While there are tangible political perks to be had — namely the fundraising dollars the campaign said it was expecting in the wake of the vote — aides and allies have long said that Trump is keenly aware of the stain impeachment leaves on his presidency. (White House senior adviser Kellyanne Conway argued to reporters Wednesday that Trump thinks it's a stain on Democrats, not himself.)

Senior campaign officials said they expected donors to contribute in blockbuster numbers on Wednesday and Thursday, in response to both the impeachment debate and campaign appeals with subject lines such as "Impeachment War."

And they expressed satisfaction with the solid House Republican opposition to impeachment, denying it the bipartisan label attached to similar action in the past.

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The White House planned to respond in real time to the impeachment proceedings throughout the day, a senior administration official said as debate began. Trump campaign officials said the president was well positioned to go to battle with his critics following progress on a trade deal with China, a new North American trade agreement and paid family leave for government employees.

Trump himself did not add any events to his schedule to address the vote, and didn't speak to reporters on his way to a campaign rally on Wednesday afternoon.

The president's most in-depth rebuttal was anticipated to take place later in the evening at that rally, where he was expected to draw from his letter to Pelosi in his remarks to supporters, with Vice President Mike Pence in attendance.