Washington (CNN) There is a common challenge confronting all of the 2020 Democratic presidential hopefuls : how to take on Donald Trump while maintaining their dignity, sanity and message control.

Whether they like it or not, Trump's would-be challengers need two strategies as their slog to the nomination begins -- one to combat their rivals for the party nomination and one to deal with the President, who is sure to insert himself into their dueling campaigns.

Monday marked a rare day when the President was not the focus of press attention -- one that found many of the Democratic rivals at competing events offering a glimpse of their differing strategies for taking him on.

For many Democratic strategists, the worst-case scenario is that which befell Sen. Elizabeth Warren of Massachusetts last year when she walked into Trump's trap and faced an unrelenting backlash by producing a DNA test that purported to show she had Native American heritage.

With the campaign in its infancy, candidates are muddling through how to best take on Trump without sinking to his level of discourse, particularly at a time when he's flinging xenophobic rhetoric about his wall in a standoff over the longest government shutdown in history.

Sen. Kamala Harris of California, who officially kicked off her campaign Monday on the symbolically resonant Martin Luther King Jr. Day, has criticized Trump's rhetoric, divisive positions and style as commander in chief. But she often does not mention his name; she speaks about restoring the "nobility of the office;" and she has so far avoided provoking him on Twitter at this early stage.

Sen. Cory Booker of New Jersey is ignoring the President. Independent Sen. Bernie Sanders of Vermont took the most aggressive approach on Monday, calling him a "racist."

After concentrating on introducing herself to voters in her first swing through Iowa, Sen. Kirsten Gillibrand of New York stepped up her attacks on Trump, accusing the President of adding fuel to a "very ugly fire" of racial, religious and community divisions.

And former Housing and Urban Development Secretary Julian Castro is presenting himself as the "antidote" to Trump.

Some in the fledgling field seem most comfortable ignoring the President as a way of avoiding his inevitable attempts to throw them off kilter and divert their efforts to keep voters focused on issues -- such as health care, economic disparity, and racial and gender equality -- that Democratic voters care about.

The downside of such a strategy is that a candidate could look weak in the face of the President's bullying, raising questions about their capacity to stand up to him in a debate or the fevered final days of the 2020 race.

Others are still deciding whether it's best to engage Trump, all the time, tweet for tweet, as a means of scoring political points.

Anti-Trump fervor

The anti-Trump feeling seething in the Democratic grass roots will create pressure on the candidates to take on the President directly.

That's one reason that Harris focuses on her toughness as a prosecutor. She underlined that point Monday when she appeared at her alma mater, Howard University.

"I love my country, and I feel a sense of responsibility to stand up and fight for the best of who we are," Harris said during her first news conference as a 2020 candidate. "I'm prepared to fight and I know how to fight."

The dangers of engaging Trump were exemplified by Warren's feeble attempt to table the controversy over her claim to Native American ancestry. She was clearly irked by the President's frequent taunting and his nickname for her: Pocahontas.

By producing a DNA test showing she could have as little as 1/1024th Native American blood, she played into Trump's attacks and distracted from her own message about the toll that economic corruption in Washington has taken on working families.

Harris has critiqued Trump's language -- his policies and his role he has played in stoking racist rhetoric from the early days of his presidency -- but she has often done so without mentioning his name.

Her announcement video was just the latest example of that fencing technique. She did not name Trump, but called on her supporters to stand with her at a time when she believes American values of justice, decency, equality, freedom and democracy are at stake.

As an implicit contrast to Trump's politics of division, she cited the aspirational message of Martin Luther King Jr. as a model Monday. And she sought to strike that kind of tone when asked why she was the best Democrat in the 2020 field to take on Trump.

Harris said Monday that she was prepared to fight for leadership that prioritizes the needs of average people, rather than self-interest. She ultimately pivoted to a message of unity without mentioning the current occupant of the White House.

"We are a diverse country, yes, and some people would suggest that in diversity, when there is a diverse population, one cannot achieve unity," Harris said. "I reject that notion, because this is my belief: Yes, we are diverse and we have so much more in common than what separates us. And when we emphasize that commonality ... we will achieve greater unity. And in particular greater unity than we have right now, when there are so many powerful voices who are trying to sow hate and division among us."

'A racist'

Booker, who said Monday that he's getting close to a decision on a run, explained why he had not mentioned Trump at all in his remarks at an MLK Day event in South Carolina, hinting at a possible campaign strategy.

"Because it's not about what we're against in life, it's about what we're for. And we need to define ourselves more about what our vision for this country is," Booker told reporters. "In all of my elections that I've had, I'm not trying to beat Republicans, I'm trying to unite Americans, because there's not a right or left way to move forward. You move forward by moving forward."

At the same event, Sanders, who is mulling a reprise of his 2016 Democratic campaign, took the opposite approach.

"It gives me no pleasure to tell you that we now have a President of the United States who is a racist," Sanders said at the rally, signaling a willingness to take on Trump over his divisive rhetoric right from the start.

Sanders is laboring to address a weakness with African-American voters, who form a critical voting block in the party's nominating electorate.

Gillibrand used her appearance at a New York MLK event to slam Trump on racial issues, possibly seeking to enhance her own credentials with African-Americans.

"Our President has chosen to tear this country apart on every line, every division, every racial line, every religious line, he chooses to divide us, community by community," Gillibrand said.

"He has inspired a hate and a darkness in this country that I have never witnessed myself. He is tearing apart the very fabric of who we are as a nation, our very common decency, and that is what we are being called to fight against," she said.

Castro is presenting himself as the anti-Trump.

"Donald Trump represents the opposite of what I am and what I believe," he said. "For many Americans, a lot changed when Donald Trump got into office. And that is what has compelled me to think about running," he told CNN in an interview this month.

2016 Republicans all failed

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No one in the 2016 Republican field was ever able to work out how to deal with Trump.

Those who initially tried to ignore him, such as Jeb Bush and Marco Rubio, struggled for traction.

When they switched strategy and tried to beat Trump at his own game, they diminished their stature against an opponent content to plunge ever deeper into the gutter.

Trump's offensive rhetoric only helped to cement his bond with his grass roots voters who loved it when he crushed political correctness and took lumps out of establishment Washington rivals.

Rubio's mockery of Trump for having small hands and his quip: "You know what they say about men with small hands," served mostly to show that "Little Marco" was not in the same league at insult politics as his tormentor.

Hillary Clinton also struggled to solve the riddle. Sometimes she ignored Trump's tweets but at others argued that his volatile temperament made him unfit for office.

"A man you can bait with a tweet is not a man we can trust with nuclear weapons," said Clinton during her convention speech.

An alternative Democratic strategy, promoted by former first lady Michelle Obama, was "when they go low, we go high."

That didn't work either.