(CNN) As the House holds its first vote Thursday to formalize the impeachment inquiry , Republican members are sticking with a messaging strategy that focuses primarily on criticizing the process -- despite a push by Donald Trump and some of his allies for a more direct defense of the President against the substance of the Democrats' charges.

Those conflicting views are adding to the frustration from other Capitol Hill Republicans at the White House's unfocused effort to guide the party through impeachment.

According to two senior aides on Capitol Hill, Republicans will continue to argue that House Democrats have conducted their impeachment inquiry into Trump's interactions with the Ukrainian government "behind closed doors" and without sufficient input from GOP members.

"You're never going to hear us stop highlighting the unfair process," a senior Republican House aide told CNN.

But as those Republicans have continued to hammer those across the aisle over what they see as an unjust process, Trump has shown he's still interested in defending himself on the merits of the case -- something on which few GOP members have focused.

The President said Monday that he had informed Republican lawmakers he wanted to discuss the allegations against him because he thought defending himself against the facts would be easy.

"And, frankly, I told Republicans, who are really being taken advantage of — they're really being maligned — and I think it's a horrible thing. ... So, one thing I said: I'd rather go into the details of the case rather than process," Trump told reporters Monday at Joint Base Andrews.

"I think you ought to look at the case," Trump added. "And the case is very simple. It's quick. It's so quick."

Trump's comments and interviews with a half dozen Republican aides and strategists reflect the ongoing disconnect between the GOP lawmakers who have spent weeks laying the groundwork for a case against the procedures House Democrats are following and a President who, many allies believe, still hasn't absorbed the seriousness of his political predicament. Amid increasing frustration from those tasked with defending him, Trump has continued to resist taking steps like creating a war room or hiring additional staff to coordinate impeachment messaging.

Republican members and aides on Capitol Hill have described their exasperation that the White House hasn't done more to coordinate their messaging with lawmakers, beyond a handful of calls between senior White House officials and conservative allies.

House Minority Leader Kevin McCarthy, a Republican from California, has distributed talking points among members for the past several weeks in attempt to create unity around the GOP's messages, but the White House has put forward an inadequate effort, these people said, to guide those who actually want to defend the President.

That's left House Republicans largely on their own.

"They're going to stick with the process arguments for as long as possible because most members still can't stomach a defense of the substance," said Brendan Buck, a former spokesman for GOP House Speakers Paul Ryan and John Boehner. "There is a real sense of inevitability to the impeachment in the House, and so the focus is on shoring up the base to minimize any Republican losses in the Senate."

GOP says they'll still be shut out

House Republicans are soldiering on with their process argument.

For weeks, leaders and backbenchers alike have blasted Democrats for not having a vote on impeachment and for holding closed-door hearings with witnesses. Despite the fact that GOP members have also sat in on those hearings, Republicans have repeatedly called for a more open process.

The release of the Democratic resolution Tuesday afternoon outlining a process -- with a vote expected by the full House on Thursday -- has not abated those complaints. House minority Whip Rep. Steve Scalise persisted in calling the inquiry a "one-sided, Soviet-style process" following the resolution's release. One senior House Republican aide called the resolution an "admission of guilt" that Democrats had been acting in bad faith but said it will not change the party's message that the process remains fundamentally flawed.

Some GOP aides and allies say the opening of the inquiry as laid out in the Democratic resolution is designed to sideline the most vocal conservative critics of impeachment by reserving public hearings for only the House Intelligence Committee.

Staunch Trump allies such as Reps. Mark Meadows of North Carolina, Jim Jordan of Ohio and Lee Zeldin of New York have held court with reporters as they walk in and out of the closed-door interviews conducted by the three committees handling the inquiry so far: Oversight, Foreign Affairs and Intelligence. But none of those three serve on Intelligence, denying them the platform to defend Trump in public hearings.

One House GOP aide told CNN the Ukraine matter hardly has an intelligence component, and that the Judiciary Committee, on which more Republicans overall and more Trump allies specifically sit, would be the better vehicle from the Republican standpoint. Two other Trump allies, Reps. Devin Nunes of California and John Ratcliffe of Texas, do sit on Intelligence and will have the opportunity to grill witnesses in public hearings.

Insufficient guidance from the White House

Although the White House began holding phone calls between senior aides and congressional allies, some Republicans have expressed concerns that Trump's team is still falling short. GOP aides and allies have been ramping up their private criticism of the way the White House is continuing to handle the impeachment process.

Some have questioned why the White House has resisted hiring additional staffers to shoulder the messaging burden, despite frequent pleas from congressional Republicans to offer more guidance for those who want to defend Trump. Several Republican aides who spoke to CNN struggled to name a specific person at the White House who is leading coordination with Capitol Hill on impeachment.

Members have been dealing with existing White House staff members, who have piled impeachment-related issues on top of their existing duties. Eric Ueland, the White House legislative director, fielded questions from reporters on Capitol Hill on Monday about the investigation even as he said he was working with lawmakers to avoid a government shutdown in roughly three weeks.

Although Trump had said just one day earlier that he wanted the focus to stay on the substance of the case, Ueland echoed congressional Republicans by attacking the procedures -- yet another sign of the White House's inability to get on the same page about an impeachment defense.

"The point the President has made repeatedly is that the process to date has been incredibly unfair," Ueland said.

Congressional Republicans have been left to navigate their way through the impeachment inquiry without much guidance from the team around Trump, leaving ample space for Democrats to dictate much of the impeachment conversation.

Without a unified rebuttal to the allegations against Trump -- that he leveraged diplomatic tools to pressure a foreign government into aiding his campaign -- Republican members have offered various responses to the underlying evidence and thus fed the perception that GOP lawmakers have no defense for the President's actions.

McCarthy, the top House Republican, has vocalized the leadership's message that the transcript of July 25 phone call between Trump and Ukrainian President Volodymyr Zelensky exonerates the American President.

"We all have the transcript. We are all able to see. There was no quid pro quo. The money was released. Ukraine did nothing and no action was taken. Where's the crime?" McCarthy said at his weekly news conference Tuesday. "What's the impeachable offense?"

But others, like Alabama Rep. Mo Brooks, have defended Trump for offering Zelensky a quid pro quo for military aid.

"Let me be real clear: In every instance in which the United States government gives another country something, whether it be military supply or it be food, there better dadgum well be a quid pro quo," Brooks said on a radio show last week in Alabama.

Despite the mixed messaging on the substance of impeachment, Republicans say the process argument has unified the House conference. GOP aides say there's still little concern there will be any defections from the ranks.

"They've actually done a pretty good job convincing base Republican voters that the system is rigged and that Democrats are just out to get him no matter what," Buck said. "I predict eventually the message will end up being that it should be up to the American people to decide the President's fate at the ballot box next year."