The West Wing portico was vacant most of Tuesday morning. The U.S. Marine who stands guard when a president is in the White House office space was not seen until 14 minutes before noon. In the meantime, President Donald Trump zeroed in on a new critic and fired away.

Cliff Sims, a former special assistant to Trump, is making the book tour rounds on cable television as he peddles the latest explosive tome about life in Trump’s West Wing. That included a stop by CNN’s “New Day” morning show Tuesday. As Sims spoke, the president appeared to be watching from the White House residence, slamming the tell-all book that describes the West Wing as a chaotic place occupied by a “team of vipers” and the man who penned it — even appearing to threaten legal action over an alleged nondisclosure agreement.

Trump dismissively called Simms a “low level staffer that I hardly knew,” saying he had authored “yet another boring book based on made up stories and fiction.” Simms “pretended to be an insider when in fact he was nothing more than a gofer. He signed a non-disclosure agreement. He is a mess!” the president added.

A low level staffer that I hardly knew named Cliff Sims wrote yet another boring book based on made up stories and fiction. He pretended to be an insider when in fact he was nothing more than a gofer. He signed a non-disclosure agreement. He is a mess!

— Donald J. Trump (@realDonaldTrump) January 29, 2019

For the commander in chief, it was another indication of how he values his “executive time” of watching cable news and attacking his political enemies. And as the 2020 Democratic field takes shape, the Republican president is sure to use his daily free time slamming potential opponents.

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One former White House official indicated that as Democratic candidates begin to campaign in earnest — and try to avoid repeating Hillary Clinton’s 2016 mistakes — Trump is likely to become increasingly fixated on their moves.

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“I don’t think Democrats are going to be stupid enough to ignore Detroit again,” said Elaine Kamarck, a former Clinton White House official now with the Brookings Institution. “These candidates aren’t going to ignore Wisconsin or Pennsylvania, either. You won’t see them doing what Hillary did, and that was focusing on algorithms rather than the actual people.”

Jim Manley, a Democratic strategist and aide to former Senate Majority Leader Harry Reid, doubts Trump will use sophisticated polling data to decide which of the expected Democratic candidates to regularly attack.

“It’ll be based on whatever he sees on cable news,” Manley said. “This is a guy who sees himself as a master strategist who can, with one tweet, move the needle and influence how the Democratic primary is going to play out. But Donald Trump isn’t going to discredit a single Democratic candidate with a single Democratic voter.”

Senior White House officials could not identify any specific system the president uses when deciding which Democratic hopeful to engage, with one saying the president himself typically makes those decisions.

But mornings and evenings offer a spotlight into whether the president will have time to keep up with the Democratic primary. He rarely arrives at the West Wing before 10 a.m., and the day’s press pool rarely has to be on White House grounds before the mid- or late-morning. Often, the Marine guard is no longer outside the West Wing entrance at 5:30 p.m. or 6 p.m. in the evening.

But the emerging Democratic battle over who will face him in the 2020 general election — unless he is defeated by a long-shot GOP primary candidate or otherwise does not run — is already on the chief executive’s mind.

Trump used part of his Monday morning “executive time” to lash out at Starbucks CEO Howard Schultz over the coffee mogul’s exploration of an independent presidential bid — challenging his intelligence and questioning his fortitude.

That came the morning after CBS’ “60 Minutes” aired a lengthy interview with Schultz, in which he said, “This president is not qualified to be the president.”

No ‘guts’?

The counterpunching president used Schultz’s critique to send a message to anyone looking to take him on, firing off a tweet declaring the Starbucks tycoon “doesn’t have the ‘guts’ to run for President!”

When “60 Minutes” interviewer Scott Pelley asked Schultz why being a “coffee entrepreneur” makes him ready to be president, the potential candidate touted his “long history of recognizing I’m not the smartest person in the room.”

He said, if elected, he would “have to recruit and attract people who are smarter than me and more experienced, more skilled” to make “great decisions about complex problems.”

Trump said he was not impressed.

“Watched him on @60Minutes last night and I agree with him that he is not the ‘smartest person,’” the president tweeted. “Besides, America already has that! I only hope that Starbucks is still paying me their rent in Trump Tower!” (Trump never completely severed direct ties with his businesses, according to Democrats and government ethics experts.)

Howard Schultz doesn’t have the “guts” to run for President! Watched him on @60Minutes last night and I agree with him that he is not the “smartest person.” Besides, America already has that! I only hope that Starbucks is still paying me their rent in Trump Tower!

— Donald J. Trump (@realDonaldTrump) January 28, 2019

Former Gov. Chris Christie, who describes himself as a “friend of the president,” on Tuesday underscored what will become a challenge for Trump as he keeps an eye and ear on the Democratic field.

“You can’t look at that group and pick out who the winner is going to be. You just can’t. And I remember being in a group in 2016 … with 17 people,” the New Jersey Republican said on Fox News of the crowded 2016 Republican field. “And at this time, three years ago … people were saying Jeb Bush was the front-runner, right? So nobody can really tell what’s going to happen with these guys.”

A new Washington Post-ABC News poll found 56 percent of Democratic voters do not yet have a preferred primary candidate.

“I always say … with presidential campaigns, when the lights go on … you either shine or you melt,” Christie added. “And we’re going to see a whole bunch of these people who think they’re going to be president melt really quick.”

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Trump’s history as a brash, elbow-throwing candidate-turned-president suggests he would like to turn the heat up on the 2020 Democratic flock. And while critics dismiss his many mocking aliases as insulting and beneath the office of the presidency, the 2016 campaign and his 2018 midterms tweets and rallies for Republicans collectively underscored how he uses his attacks on rivals and foes to fire up his conservative base — and plant seeds of doubt about his opponents.

Mum on Harris

As of press time, however, Trump had not weighed in on a candidate many political analysts say might emerge as one of the Democrats’ top White House contenders: California Sen. Kamala Harris, a former state attorney general and San Francisco district attorney.

Even as cable news chattered Monday and into Tuesday about the estimated 20,000 people who attended her campaign kickoff event Sunday in her native Oakland, Trump held his fire.

In fact, Trump has never tweeted about the junior senator from California, according to an online archive of every post on the social media site since he opened his account in March 2009.

“I certainly think it’ll come at some point,” Manley said of a presidential tweet or nickname for Harris. “He’s probably busy thinking about who to bestow a nickname on, and thinking about what he thinks would stick and do the most damage.

“I certainly don’t think the silence is because she’s an African-American or a woman,” Manley said. “This is a guy who goes with his gut. So if he does view her as a threat, I don’t see him holding back.”

Making his silence on Harris more intriguing is his fixation on crowd size. On his first day in office, Trump dispatched Press Secretary Sean Spicer to the White House briefing room to loudly tick off a list of false claims about his inauguration crowd on the National Mall.

Most recently, local media outlets have found no evidence to support Trump’s assertions that tens of thousands of people were turned away from his 2018 rallies and watching outside on large television screens.

Harris also took several shots at the White House’s current occupant and his brash, personal style of politics in her Sunday remarks.

“We are here because the American dream and our American democracy are under attack and on the line like never before,” she said. “And we are here at this moment in time because we must answer a fundamental question: ‘Who are we? Who are we as Americans?’ So, let’s answer that question to the world and each other, right here and right now: ‘America, we are better than this.’”