ABC News(WASHINGTON) -- As the House impeachment probe moves toward a new phase of open hearings, President Donald Trump is calling on Republican lawmakers to shift their strategy from attacking the process of the inquiry to poking holes in the substance of House Democrats' case.

"I'd rather go into the details of the case, rather than process," Trump said to reporters on Monday. He later added, "Process is good. But I think you oughta look at the case."

On Wednesday, Trump repeated that sentiment on Twitter.

"Republicans are very unified and energized in our fight on the Impeachment Hoax with the Do Nothing Democrats, and now are starting to go after the Substance even more than the very infair (sic) Process because just a casual reading of the Transcript leads EVERYBODY to see that....." Trump tweeted.

He continued the thread, tweeting, ".....the call with the Ukrainian President was a totally appropriate one. As he said, ‘No Pressure.' This Impeachment nonsense is just a continuation of the Witch Hunt Hoax, which has been going on since before I even got elected. Rupublicans (sic), go with Substance and close it out!"

But with the House set to vote Thursday on a resolution outlining the next steps of the impeachment inquiry, many Republicans are still crying foul over how the Democrats are approaching the inquiry.

In fact, even White House press secretary Stephanie Grisham released a statement responding to the draft resolution and describing the inquiry as "an illegitimate scam."

"The resolution put forward by Speaker Pelosi confirms that House Democrats' impeachment has been an illegitimate sham from the start as it lacked any proper authorization by a House vote," Grisham said.

When asked about the president encouraging his allies to direct their criticism toward the substance of the impeachment inquiry rather than the process, Rep. Matt Gaetz, R-Fla., told ABC News that Republicans can do both.

"Look we can make our substantive arguments, while at the same time pointing out how unfair Democrats are treating the president and treating House Republicans," said Gaetz, a staunch Trump ally who led a group of Republicans who stormed a closed-door hearing last week.

Gaetz is also filing an ethics complaint against House Intelligence Committee Chairman Rep. Adam Schiff, D-Calif., who is leading the inquiry.

"When you make an ethics complaint, you're really speaking on behalf of the institution, to try to raise the conduct of the members of the institution," Gaetz said.

In response to the criticism, Schiff tweeted his own reply.

"Trump has urged Republicans to focus on the substance in the impeachment inquiry, not the process," Schiff tweeted. "I can confirm our focus will continue to be on the President's own words and misconduct. Glad we all agree."

Despite the president's urging, Senate Majority Leader Mitch McConnell, R-Ky., refused to comment on the substance of the inquiry on Tuesday.

"I'm not going to comment on the merits of what's going forward. We're watching what happens in the House, and we'll see whether they actually open this impeachment inquiry," McConnell told ABC News. "Apparently they're gonna vote on it later this week; it's going to be a very interesting vote."

On Wednesday, McConnell took his criticism of the impeachment inquiry to the Senate floor.

"The resolution merely seems to contemplate that maybe, maybe someday in the future at some other phase of this, due process might -- might -- finally kick in," McConnell said. "But only if the House Judiciary Committee feels like holding hearings and calling its own witnesses. In other words, no due process now, maybe some later, but only if we feel like it."

Further decrying the process, six Republican representatives from Michigan sent a letter to Pelosi Wednesday afternoon saying the proceedings fall "far short" of the "bedrock principle of due process." The group admits in the letter they "do not presume to know all that the impeachment inquiry may find," but are certain "a more transparent process that is beyond reproach can only lead to a more credible outcome."

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