Once, the President's uncanny sense for weakness in others helped him destroy the deepest Republican primary field in history. Now, the suddenly quickening 2020 presidential race and a Capitol Hill power play by Democrats suggest that Trump is beginning to take on the look of prey for opponents ready to pounce on his sub-40% approval rating.

Democratic presidential candidate Sen. Kamala Harris focused directly on Trump's character on the second day of her campaign, during a CNN town hall event in Iowa on Monday.

"It's very important that anyone who presents themselves as a leader and wants to be a leader will speak like a leader," Harris said, not mentioning the President by name but leaving no doubt about her target.

"That means speaking with integrity. It means speaking truth. It means speaking in a way that expresses and indicates some level of interest and concern to people other than oneself. And so, right there we will see a great contrast."

Washington is a place where the merest hint of vulnerability never goes unnoticed. That, paired with the clearest signs yet that the Russia investigation is coming to a close , including reverberations from the indictment of Trump's political guru Roger Stone, is deepening impressions of a White House under siege.

Trump showed himself in 2016 to be a formidable campaigner and found a path to the White House that many observers thought was impossible. But as the 2020 race accelerates he is in a tenuous position that means the Democratic nomination is an especially attractive prize.

A Washington Post-ABC News poll just published showed that 56% of registered voters say they will "definitely" not vote for Trump next year. While those numbers can change, they are clearly a problem for the President, even though he won last time with only 46% of the popular vote.

It was not supposed to be this way for Trump -- not on this Tuesday anyway.

He should have been astride the national stage in his State of the Union address, a president's best chance each year to use the pageantry of his position and a huge TV audience to make his case.

But Trump was told during the shutdown last week by House Speaker Nancy Pelosi, in a blatant demonstration of Washington's new power dynamics, not to show up.

He will get to give his big speech next week -- but the President's shutdown cave on Friday means he will not be quite so feared when he finally steps up in the House of Representatives.

Political vulnerability is a new feeling for the President.

From the start of his presidential campaign he was a front-runner -- forcing his rivals to react to his antics and unpredictable shifts that tore up the playbook of conventional politics.

Even as his presidency has lurched from crisis to crisis and he has failed to command majority support in the country, Trump has always dictated events -- with unexpected moves such as his summit with North Korean leader Kim Jong Un or with tweets that put the nation and the world on edge.

But in the aftermath of the shutdown and a midterm election rout, Trump is under scrutiny to see if he can stage the kind of political comeback that is crucial to every successful president.

While it is far too early to write off Trump's political acumen, he has work to do.

After all, he risked his standing with his conservative base by ending the shutdown without forcing Pelosi to fund the border wall. And he cemented Democrats behind their leader after handing her victory.

That left him needing to wrest back control of the political agenda from rivals who are becoming increasingly successful at pegging him back.

It will be a test of political skill on which his presidency may depend.

Sharing the spotlight

For the last two years, Trump has been the undisputed star of the reality show with which he's replaced conventional Washington politics. But he is now going to have to share the stage.

Pelosi waited just long enough to reissue her invitation to Trump to deliver the State of the Union address to stress the institutional power over the President: The speech is now set for next Tuesday.

Trump has also ceded some of the spotlight to his would-be 2020 rivals.

Harris made a splash over the weekend with a rally before a big, pumped-up crowd in her home state of California. The event was both a projection of strength to her opponents and a sign that she believed there was a path to victory by presenting Trump -- whom she did not mention by name -- as the antithesis of everything America stands for.

"We are at an inflection point in the history of our nation," Harris said.

Harris is not the only Democratic presidential candidate to test out campaign themes that could exploit the fervor in their party to oust Trump, and could be used against the President in a head-to-head clash.

New York Sen. Kirsten Gillibrand is attacking Trump for his "very ugly fire" of racial rhetoric. And like Harris, Texas Democrat Julian Castro is promoting himself as the "antidote" to the President.

And the possibility that Trump could be vulnerable in his re-election year is attracting interest outside the Democratic Party.

Coffee mogul Howard Schultz also thinks his fellow billionaire is so wobbly that he could be toppled by an independent candidacy, a path that has traditionally been difficult given the hold of the two major parties on power.

"I think, like most people ... I've become bored with President Trump and his tweets," Schultz told CBS News' "60 Minutes" in an interview aired Sunday.

All of this is a sign that Trump may need to go back to where he performs best -- the campaign trail -- where he can sharpen his counterattacks and try to lure his rivals into bottom-of-the-barrel fights in which he has no political equal.

But the President may soon have more to worry about than the election.

Acting Attorney General Matt Whitaker on Monday revived anticipation over the outcome of the Mueller probe, saying he thinks it is "close to being completed."

And Trump's former lawyer Michael Cohen on Monday agreed to testify in private to the House Intelligence Committee next week.

His testimony in public before another House committee, from which he backed away last week, fearing for the security of his family, might also be back on, according to his new lawyer Michael Monico.

New, public revelations about Trump's business and personal relationships before he became President would be almost certain to set the political vultures swirling around the White House again.

Show of force

On Monday, it was as if Trump's West Wing sensed an urgent need for a show of force, as Washington digests the aftermath of the 35-day shutdown, which ended with the President as far away as ever from getting his border wall.

This was a ship that needed steadying.

The administration suddenly rediscovered the White House press briefing room and rolled out Treasury Secretary Steven Mnuchin, national security adviser John Bolton and top economic adviser Larry Kudlow.

Bolton and Mnuchin announced extremely strong action designed to tip the balance in Venezuela's political crisis , and the treasury secretary hinted at a possible breakthrough in talks with China to avert a trade war.

Kudlow also sought to give the impression of a vigorous White House, disputing various assessments that the President had dented the economy, his most convincing argument for re-election in 2020, by leading the country into a futile shutdown.

"The switch goes right back on," Kudlow said.

And with speculation swirling that special counsel Robert Mueller's final report could be devastating for Trump, press secretary Sarah Sanders sought to stamp out any sense that Trump's presidency is facing an existential threat.

"Not at all. In fact, I think nothing could be further from the truth," Sanders told CNN's Jim Acosta in her first briefing in 41 days.