(CNN) President Donald Trump is carving new divides on immigration ahead of his State of the Union address that likely offer a more reliable guide to the year ahead than the ritual calls for national unity he is expected to issue on Tuesday night.

"With Caravans marching through Mexico and toward our Country, Republicans must be prepared to do whatever is necessary for STRONG Border Security," Trump wrote in a tweet on Sunday evening.

"Dems do nothing. If there is no Wall, there is no Security. Human Trafficking, Drugs and Criminals of all dimensions - KEEP OUT!"

The Twitter blast shattered the White House spin that Trump is intent on healing old wounds, reaching across divides and using showpiece annual address before a vast television audience to project optimism.

The timing of Trump's tweet may be no coincidence -- it emerged shortly after Axios published weeks of leaked West Wing schedules painting an unflattering picture of a President who spends hours a day in unstructured "executive time."

But Trump's attempts to distract are often instructive since they usually revive the scorched earth style of politics that fired up his grassroots supporters in 2016 and can explain why he has rarely, if ever, attracted majority political support.

That's one reason why it will be important to watch how Trump behaves before and after his State of the Union address -- rather than just the tone of his scripted remarks as he relishes the spotlight of a prime-time television audience.

The obvious political play on Tuesday for a commander in chief who is perpetually underwater would be clear to any other President than Trump.

With his approval ratings dipping after a humiliating defeat over the government shutdown and with the Russia scandal closing in on his White House, a conventional president would stress unity and seek to widen his support.

At a time of jarring national political divides, he might seek to shift to the political center as Democrats move left at the start of their 2020 primary campaign and to position himself to widen the narrow path to 270 electoral voters he navigated in 2016.

Such a president would be sure to spend most of his address on the roaring economy and stress shiny job creation numbers which make the best argument for his re-election. In order to avoid tainting his big moment with partisan jabs, he might steer clear of his hardline views on the most divisive issues. In Trump's case that might mean softened rhetoric on immigration — since most polls show he has the support of less than half the country on the issue.

But that's not how Trump rolls.

The President who built a political career on mobilizing a faithful, angry base and deliberately tweaking the nation's social and cultural divides is unlikely to signal the fundamental change of direction on Tuesday night that his perilous political plight might suggest.

While Trump will stand up in the House of Representatives and call on the nation to unite and bury its differences and is likely to effectively leverage presidential pageantry, the real test comes in the days after Tuesday's address.

History suggests that however inspiring his prime-time calls to American greatness and common purpose, he will be back to setting Twitter aflame with personal attacks and stoking outrage fairly swiftly.

Yet within days, prime-time Trump was nowhere to be seen.

Days after his 2017 speech, he accused former President Barack Obama of wiretapping him in Trump Tower during the election, an accusation that lacked proof but caused partisan uproar in Washington.

So on the principle of twice bitten, three times shy, it might be best to wait a few weeks to digest the true impact of Trump's speech this year.

Trump will seek to inspire

Still, Trump is nothing but unpredictable.

A senior administration official tried to explain to reporters on Friday how such a tone would not come across as incompatible with the President's normal combative rhetoric.

"Well the State of the Union is unlike other occasions in the year. And one tries to, in that occasion, when you have 60 -- 60 to 70 unfiltered minutes to the American public you get a chance to explain your whole agenda," the official said.

"And you get to strike the tone and optimism that you think is appropriate in that venue."

With that in mind, Trump will say, according to an excerpt: "Together, we can break decades of political stalemate, we can bridge old divisions, heal old wounds, build new coalitions, forge new solutions, and unlock the extraordinary promise of America's future."

The speech will address issues of concern to all Americans, like the cost of prescription drugs, infrastructure and national security — a section that is likely to include mention of the rising US role in Venezuela's political meltdown.

Trump has already trawled a possible announcement on a second summit with North Korean leader Kim Jong Un — which should be cause for bipartisan hope, given the threat posed by Pyongyang's nuclear arsenal. Yet top intelligence chiefs last week all but repudiated Trump's optimism that a deal is possible with the isolated state.

Skeptics might also note that Trump called for a major infrastructure push with Democrats in last year's State of the Union, but little happened.

The speech is also likely to contain material that will overshadow any conciliatory moves by the President.

Judging by his own teases over the last few days, he has no plan to steer clear of his demand for funding for his border wall as a deadline looms 12 days away that threatens to close down the government again.

Trump last week said he regarded talks between Democrats and Republicans in the House and the Senate on a compromise funding package to keep government open after a February 15 deadline as a "waste of time."

And he hinted that he would use the State of the Union to signal that he would move ahead with executive action — possible a declaration of national emergency to get the wall built.

"I don't want to say. But you'll hear the State of the Union and then you'll see what happens right after the State of the Union, OK?" the President said on Friday.

One way to fracture national unity: build the wall

If Trump does intend to move ahead and use executive power to build the border wall that was the emotional center of his appeal to voters in 2016, he would immediately undermine any calls to unity in his speech.

new CBS poll Sunday found that a majority of Americans — 66% — say Trump should not declare a national emergency to build the wall if Congress refuses to fund it. And 73% of those polled want Trump to continue negotiating while keeping the government open if he doesn't get wall money by February 15.

If he were to keep Americans united on immigration, he would likely not press ahead.

The Washington Post reported meanwhile that Senate Majority leader Mitch McConnell has told the President that a declaration of national emergency would threaten unity in the GOP coalition itself.

And a declaration of national emergency would represent one of the most sweeping attempts to use executive power since President Harry S. Truman ordered the nationalization of steel mills during the Korean War.

Yet Trump has a political rationale for going ahead anyway.

He's often tested the limits of his power during two years in the White House.

And the CBS poll showed that while a majority of Americans oppose an emergency declaration to build a wall if Congress doesn't fund it — 73% of Republicans favor it.

Trump is also aggressively marketing his speech — delayed a week after House Speaker Nancy Pelosi said he could not give it during the shutdown — to his supporters.

"Over the past few weeks, (Democrats) have taken their obstruction and radicalism to a whole new UN-AMERICAN level," Trump said in an email to supporters Sunday.

"First, they REFUSED to put the safety of Americans at our Southern Border above their own political interests, and then they disinvited me from speaking at the PEOPLE'S State of the Union on the original date."

Trump has rarely made much effort to broaden his political coalition in office — but has shown intense interest in his support among his political base and the loudest of conservative commentators — politically on immigration.

So if there is a choice between living up to his pleas for national unity on Tuesday night and consolidating his support among the voters that like his presidency the most, it's not hard to work out which way he's likely to go.