WASHINGTON — It’s hard to create a war room without a reliable warrior.

That’s the reality that has stymied allies of President Donald Trump over the past week as they’ve tried to convince the president to implement a strategy for countering the impeachment inquiry that’s engulfing his White House and some of the top members of his Cabinet.

Instead, White House officials are scrambling to come up with a political strategy that allows for a president who at any moment could undermine or overrule that plan without warning or consultation, according to administration officials and White House allies.

“The plan is, whatever the president’s whim is today,” one person close to the president said.

Over the past week, Trump’s advisers have held a series of meetings in an attempt to cobble together a cohesive approach — and each day, the president has made statements that shifted the parameters of the impeachment inquiry in unexpected ways and in real time.

On Friday, Trump appeared to unveil yet another explanation for why he’s been asking foreign leaders to investigate a political rival, former Vice President Joe Biden, and his son Hunter, saying it wasn't because he thought the elder Biden might be the Democratic nominee, but because it’s his obligation to ensure investigations into alleged corrupt behavior by Americans with business dealings outside the United States.

“I’m only interested in corruption. I don’t care about politics. I don’t care about Biden’s politics,” Trump told reporters. “I never thought Biden was going to win. I don’t care about politics. But I do care about corruption, and this whole thing is about corruption.”

He also sought to co-opt Democratic messaging — that it’s their constitutional duty to look into the president’s requests for a foreign government to investigate a political rival — by saying his actions had reflected his own duty to the Constitution.

“As President I have an obligation to end CORRUPTION, even if that means requesting the help of a foreign country or countries,” Trump wrote on Twitter.

Download the NBC News app for full coverage of the impeachment inquiry

Acting White House chief of staff Mick Mulvaney and the president’s son-in-law and senior White House adviser Jared Kushner are trying to take the lead on the West Wing’s response to the impeachment inquiry, officials said. Mulvaney has led several meetings this week with White House officials on impeachment, which have included Kushner, a key conduit with the president’s re-election campaign.

The Trump campaign has rolled out three impeachment-related ads in the last week, all echoing Trump’s language on “corruption” and accusing Democrats of leading a “coup” to remove him from office.

As Mulvaney and Kushner have worked to streamline public responses to the impeachment inquiry by everyone except the president, they are also trying to coordinate across government agencies and take into account actions by the state and defense departments that are out of their control, according to one official.

The Pentagon and the State Department have their own inspectors general, and some of their employees may be required to cooperate with any internal investigations and Congress, the official said. On Thursday, the Pentagon announced that its general counsel had directed all Defense Department offices to produce information dealing with the delayed military funding for Ukraine.

Trump allies outside the administration have expressed frustration at the lack of a clear messaging strategy from the White House, leaving them to “muddy the waters as much as possible” until a well-defined defense emerges, officials said.

They’ve also suggested Trump has been dismissive of his allies both inside and outside the White House and on Capitol Hill, who have tried to convince him to stick to a messaging plan. “The president doesn’t want to hear a contrarian view,” the person close to the president said.

Part of the headwinds facing those around the president who favor a clear strategy and messaging plan is that some people close to him have bolstered his belief that he doesn’t need one. They have pointed out polling that shows his unwavering support from Republicans, and rising fundraising numbers since Democrats announced the impeachment inquiry would get underway, this person said.

One administration official described the White House strategy meetings on impeachment this week as routine, saying they’re little more than addenda to normally scheduled policy sessions.

“We’re treating it like any other issue, [such as] health care or immigration, “ the official added.

The meetings included people from the White House counsel’s office, the legislative affairs office and the communications team. One challenge for the group is coordinating the daily message with surrogates, who are supposed to coordinate a unified Republican response to the Democrats' impeachment inquiry, given fast-moving revelations and an unpredictable president, one administration official and one outside adviser said.

Late Thursday, for instance, congressional investigators released text messages that showed U.S. diplomats pressuring Ukraine to commit in public to do an investigation that would benefit Trump and linking that inquiry to a potential White House visit by Ukraine’s president. The two ambassadors, both Trump picks, went so far as to draft language for what Ukrainian President Volodymyr Zelenskiy should say, the texts indicated.

Those revelations followed Trump’s call just hours earlier for China to look into the foreign business dealings of Hunter Biden, adding to the actions his allies were forced to defend. “China should investigate the Bidens,” Trump said Thursday. “China is just about as bad as what happened with Ukraine.”