(CNN) Donald Trump just co-opted a new buzz phrase he hopes will define the next two years in politics: "Presidential harassment."

His jab at the tactics of the incoming Democratic House represents an early effort to spin a new era of investigations and oversight that is about to shake the White House as a power grab by his opposition.

Trump's appropriation on Twitter of a concept first coined by Senate Majority Leader Mitch McConnell last week , points to the critical nature of the fight the President must wage to safeguard his hold on power, one that will surely start to feel pressure as lawmakers return to Capitol Hill with newly-elected members in tow.

Trump's comment is also a reminder to Democrats that any sign they are persecuting the President without grounds, or acting unfairly could harm them with moderate voters who swapped sides in the midterm elections.

"The prospect of Presidential Harassment by the Dems is causing the Stock Market big headaches!" Trump tweeted Monday, falsely blaming his foes for a sell-off motivated by jitters in the tech sector.

The prospect of Presidential Harassment by the Dems is causing the Stock Market big headaches! — Donald J. Trump (@realDonaldTrump) November 12, 2018

But the President's intervention did underline how Washington's calendar seems to have already flipped to 2019.

Trump and Democratic leaders are making calculations and staking out positions that will shape the new political environment, set the stage for the 2020 election, and could ultimately dictate Trump's political fate.

The arrival of a new class of lawmakers for orientation Tuesday will exacerbate the feeling of a fresh start, though critical business remains in the lame duck twilight of the GOP-led House — that could even degenerate into a government shutdown before year's end.

On talk shows in recent days, incoming Democratic committee chairmen fleshed out earlier promises to look into Trump's policies and personal affairs.

At the same time, the likely next House Speaker Nancy Pelosi is trying to keep talk of impeachment f rom bubbling over.

Some Washington veterans are already seeking parallels from the past, including the period after the 1994 midterms, when a new GOP majority that came to be seen as overzealous gave President Bill Clinton a foil that he used to win re-election two years on.

With that in mind, House Democrats know they have to do enough to appease their voters outraged by Trump, 77% of whom, according to exit polls, support impeachment.

But moving too fast could harm the credibility of Democratic oversight and foster an impression it is motivated by purely political motives rather than being in the national interest, and could offer Trump an opening.

The party must also keep faith with the policy agenda that helped them win last week. That involves passing bills on securing coverage for Americans with pre-existing health conditions, acting to control the cost of college, and possibly working on infrastructure, even if most such measures hit a brick wall in the GOP Senate.

"I think the key word is balance in this," said Jim Kessler, senior vice president for policy at Third Way, a center-left think tank.

"There needs to be aggressive oversight of this administration," Kessler said. "There has been no oversight for two years. I think the watchword would be -- make these investigations in the taxpayers' interests."

California Rep. Adam Schiff, the incoming House Intelligence Committee chairman, one of Trump's most aggressive critics on Russia, says the political delicacy of the coming clash is one reason Democrats should retain Pelosi as leader, and speaker, even if more radical members of the new class want change.

"We need the strongest general that we have," Schiff said on NBC's "Meet the Press" over the weekend. "We need the best tactician, we need the best organizer."

Pelosi wants her party to stay unified and temperate as they erect the new checks and balances against the President.

"We are responsible. We are not scatter shot. We are not doing any investigation for a political purpose but to seek the truth," she said on CBS' "Face the Nation" on Sunday, a message she repeated in a letter to members Monday

"I think a word that you could describe about how Democrats will go forward in this regard is we will be very strategic," she told CBS.

Trump's new reality

The President has been in a dark place since election day, as Democratic gains in slow moving counts underscored his rebuke by voters.

He and his top officials will soon be facing Democratic probes, hearings and potentially subpoenas on everything from his unreleased tax returns to the family separations storm, from his alleged obstruction of special counsel Robert Mueller's probe to his family finances.

He is already seeking to brand Democrats as a party obsessed with attacking him, while ignoring the challenges he believes many Americans care about, like the humming economy and what he sees as an immigration crisis.

He is leaving open the door to cooperation on some issues but warning that once Democrats crank up investigations, he is ready to use his power to target them, and grind government to a halt.

"I would blame them because they now are going to be coming up with policy. They're the majority in the House," the President said at a post midterm election news conference last week.

Trump's first goal is to curate the battlefield ahead of his re-election bid in 2020 after Democratic gains repaired gaps in the party's blue wall in the Midwest that Trump tore down in 2016.

But, with the possibility of impeachment looming, depending on the contents of Mueller's final report, the President knows he must also act to shore up his support base in Congress, especially in the Senate.

There is no sign at this stage that there would be a two-thirds majority in the GOP-led chamber to convict him after any successful impeachment push in the House -- a factor that may convince Democratic leaders to avoid such a route.

But the most dangerous threat to his presidency -- as unlikely as it seems now -- would lie in any gathering political weakness that would erode his support among Senate Republicans.

After all, President Richard Nixon resigned during Watergate not after admitting guilt, but after concluding that he no longer had "strong enough political base in the Congress to justify continuing" in office.

Trump's desire to avoid a similar fate seems to lie behind his own obsessive efforts to bolster his political "base" and to show Republican lawmakers they would face a terrible price in their districts from deserting him. It also helps explain the aggressive campaign to discredit Mueller's "witch hunt" undertaken with the enthusiastic aid of conservative media.

The opening shots between Trump and resurgent Democrats show that the political battle between each end of Pennsylvania Avenue will be bitter and prolonged.

At the outset of this existential clash, it is less clear whether, given the polarization in Congress and the nation, there is any potential resolution, however it turns out, that will spare the stressed political system even more blows to its tattered legitimacy.