President Trump knocked the House Judiciary Committee late Tuesday for allowing one of former special counsel Robert Mueller's deputies to accompany him during his testimony Wednesday, saying he should not be able to appear for the hearing.

"Just got back only to hear of a last minute change allowing a Never Trumper attorney to help Robert Mueller with his testimony before Congress tomorrow. What a disgrace to our system. Never heard of this before," Trump tweeted.

"VERY UNFAIR, SHOULD NOT BE ALLOWED. A rigged Witch Hunt!" he added.

The Judiciary panel said earlier in the day it would permit Aaron Zebley to join Wednesday's hearing as Mueller's counsel, a change announced less than 24 hours before the former special counsel's blockbuster testimony is set to begin.

A Republican source told The Hill earlier Tuesday that Mueller had requested that Zebley join him.

Rep. Doug Collins (Ga.), the top Republican on the committee, criticized Judiciary Committee Chairman Jerrold Nadler (D-N.Y.) over the move, asserting it would "jeopardize whether tomorrow's hearing complies with the rules of the House."

Trump's ire toward Zebley is likely sparked by the fact that he represented the IT aide who helped Hillary Clinton set up a private email server.

"His lawyer represented the "basement server guy" who got off free in the Crooked Hillary case. This should NOT be allowed," Trump said in a subsequent tweet.

Republicans first cast Zebley's presence as an attempt to add a second witness to the hearing, which they said would run afoul of House rules.

"You don't get to change the rules right before kickoff, especially after a 22 month, $30 million investigation," tweeted Rep. Jim Jordan (R-Ohio), a member of the Judiciary panel and a chief attack dog on behalf of the White House.

Zebley is not expected to testify and is instead expected to advise his former boss as he faces an avalanche of questions over his two-year probe into Russia's election meddling and obstruction of justice. Zebley worked closely with Mueller during the special counsel investigation and was chief of staff to Mueller during his time heading the FBI.

Mueller is scheduled to testify in back-to-back hearings before the House Judiciary and Intelligence committees beginning at 8:30 a.m.