Washington (CNN) A fateful confrontation for the ages is fast approaching over the limits of presidential authority between Donald Trump and the political, investigative and judicial bodies that stand in his way.

The three top intrigues consuming Washington -- the Russia probe , the duel over the border wall and Trump's battle with the new Democratic-led House -- all ultimately revolve around his grabs for authority and efforts to constrain him.

Trump, special counsel Robert Mueller and House Democrats are arming themselves for an almighty political and constitutional tussle that will define the coming year, and possibly the current presidency itself.

At stake is the system's capacity to rein in an aggressive President, which will inevitably be seen by his fervent base as an attempt by elites to subvert a democratic election.

If the contentious first few weeks of 2019 are any indication, the coming year will see Trump and counterweight forces in the US political system square off, as the partial government shutdown over the wall intensifies, Mueller inexorably marches toward the end of his investigation and Democrats flex muscle.

The early stages of this skirmish were evident Thursday in signs the White House is preparing to assert an aggressive claim of executive privilege in the hope of preventing the public release of large swathes of Mueller's final report.

The White House counsel's office is staffing up for the fight. Trump, meanwhile, appears to be moving toward declaring a national emergency on the southern border in order to bypass Congress and build his wall.

Such a move would unleash a fresh constitutional argument and is likely to infuriate the President if the courts, as expected, step in to block his power play, igniting a long legal battle.

Trump signaled he was ready to move ahead after the failure of the latest talks with congressional Democrats on solving the government shutdown.

"If this doesn't work out, probably I will do it, I would almost say definitely," Trump said. "This is a national emergency."

For their part, House Democrats are taking the first steps in what will become a punishing oversight campaign in announcing that Michael Cohen , the former personal attorney who flipped on Trump, would testify in public next month.

Balance of power

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Tension among the three branches of government is a built-in feature of the US system, and presidents from John Adams to Barack Obama have chafed at constraints on their power.

But it is difficult to point to any time in the modern era when so many of the issues driving the national debate revolve around what a President can legally do, and the effectiveness of competing power bases in the federal system.

Some of this can be put down to the character of Trump himself. The President is mercurial and unruly. He's an outsider unfamiliar with the institutional constraints of the US constitutional system. His outlook was shaped in a family business where he enjoyed absolute power, and he sought to transfer his method to politics.

In two years in office, Trump has torn at historical norms and stamped over protocol, railing at the Washington establishment. Such behavior is why his millions of supporters stick by him still, but it repeatedly brings him into conflict with the edifices of political and legal accountability.

He's frequently butted up against the courts, which have frustrated his most sweeping previous efforts to overhaul the immigration system. Trump has flailed away at the invisible wall meant to separate the White House from the Justice Department. Former top officials have said they often had to ensure t he President did not exceed his authority under the law.

A war over executive privilege

It has long been speculated that the end of the special counsel probe would kick off a bruising constitutional fight about the destiny of a final report -- even before any possible impeachment proceedings in the Democratic-led House.

That scenario is now all but inevitable.

CNN reported on Thursday that there is a building effort behind the scenes in the White House to keep a lot of what Mueller submits private . New White House Counsel Pat Cipollone has hired 17 new lawyers, a senior administration official said. Trump's legal team is preparing to argue that a large part of the report should be protected by executive privilege, the custom that says conversations between a President and his advisers should be private.

"Executive privilege is a term that we all should get familiar with because we are going to hear it a lot in 2019, I think more than any year since 1974, when Richard Nixon was up on the ropes and invoked executive privilege, unsuccessfully," said Elie Honig, a former federal prosecutor who's a CNN analyst.

The Nixon precedent could be a troubling one for the current President. The 37th president claimed executive privilege to try to avoid turning over White House tapes and other subpoenaed materials. But the Supreme Court ruled that executive privilege was limited to communications linked to military and diplomatic issues and did not provide a shield for the president against the demands of due process and the fair administration of justice.

Case law on executive privilege is limited, and many constitutional experts expect that any such claims made by the Trump administration would precipitate an epochal Supreme Court case.

CNN also reported on Thursday that Mueller was focusing on conflicting public statements by Trump and his team that could be seen as an effort to influence witnesses and obstruct justice.

Mueller's appointment came about only because of an uproar over the firing of former FBI Director James Comey, which soon became the centerpiece of the investigation into whether Trump had obstructed justice. The President appeared to say on NBC News that he had fired Comey because he had been investigating alleged collusion between Trump's campaign and Russia in 2016.

Some of Trump's defenders argue that it is impossible for a president to obstruct justice because ultimately he is in charge of the Justice Department.

His lawyer Rudy Giuliani did not go that far in an interview with CNN's Jake Tapper last August but sketched a possible defense, pointing out that the President has the power to dismiss executive branch employees.

"When he's exercising his power as President ... exercising the power of a president -- and he's firing somebody, then it becomes very, very questionable whether it can be an obstruction of justice," Giuliani said.

Experts who reject that view argue that if a president is proved to be acting with corrupt intent, to cover up a crime or thwart a criminal investigation into his own conduct, he can be guilty of obstruction.

Democrats get ready

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While Trump girds for a showdown, Democrats are also springing into action.

House Ethics Chairman Ted Deutch told CNN's Wolf Blitzer on Thursday that the party's House majority would resist any effort by the White House to hide key details in the Mueller report.

"There is no executive privilege over presidential deliberations with the respect to possible obstruction of justice, no executive privilege to hide potential witness tampering, or the potential cover-up of the commission of a federal crime in order to get elected president of the United States," the Florida Democrat said.

"What we are going to do is make sure that the President, and his lawyers, however many lawyers there are, can't be allowed to proceed with arguments that aren't permitted by law," he said.

The struggle over executive power is also likely to play out when House Democratic chairmen start flinging subpoenas toward top White House officials, a move likely to trigger new privilege clashes.

The most glaring sign so far of the new Democratic House's capacity to hurt Trump will come when Cohen testifies in public before the Oversight Committee on February 7.

Cohen will not talk about Russia, to avoid damaging Mueller's investigation, with which he has been cooperating following his conviction on tax and fraud charges in New York. But he could potentially shed light on his work for Trump before he was elected, including the campaign finance crime to which Cohen confessed and in which prosecutors indirectly implicated the President.

Republicans will try to impugn Cohen as a witness. But the hearing promises to be the most compelling Capitol Hill appearance from one of the President's men since Nixon's former White House counsel John Dean testified about Watergate more than 40 years ago.

Opinion is divided in Washington on whether the President can use executive power to go around Congress and build the wall. Some scholars believe he does have the authority to reprogram existing Pentagon funds. But any future funding would still need to be appropriated by Congress.

The dispute, likely to quickly end up in the courts, also concerns presidential power since it could establish a precedent that could echo for generations if a commander in chief who sees his top political priority blocked by Congress can simply grant himself the authority to carry it out unilaterally.