Two minutes into his stump speech in the strongly Democratic college town of Iowa City, Independent Senator Bernie Sanders made his first of many references to President Donald Trump, calling him "the most dangerous President in modern American history".

A short time later, he told the crowd that Trump "embarrasses us every day", before attacking the President's healthcare promises. "I know it will shock you when I tell you he lied," Sanders added.

Then, at an appearance in New Hampshire, he told the crowd how his governing style would differ from Trump's. "The underlying principles of our Government will not be racism, sexism, homophobia and religious bigotry," he said. "The principles that our Government will work on will be based on justice."

In his first swing through Iowa and New Hampshire, Sanders has been an anomaly among the Democrats vying for the White House, as he routinely uttered two words that other candidates have often avoided: "Donald Trump." The President dominates Twitter feeds, news cycles and the political landscape in almost every corner of the country — except venues where Democrats with presidential aspirations are speaking to voters. Those Democrats are strategising about how or whether to attack Trump explicitly, and in a twist, many are uttering his name rarely, if at all.

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Given Democrats' anger at Trump, it might seem natural for the party's hopefuls to frequently give voice to that fury. But most have concluded that they must show they can rise above ferocious attacks, especially if they hope to carry their message to a general election where not everyone shares their sentiments.

It's also an attempt to learn the lessons of the 2018 midterm elections, when victorious Democrats, especially in swing districts, generally focused on issues such as healthcare, education and job security, which polls show voters care deeply about.

Recent history suggests that getting into a verbal war with Trump often plays into his hands, as reflected in the fate of 2016 GOP rivals such as senators Marco Rubio, whom Trump dubbed "Little Marco", and Ted Cruz, who received the moniker "Lyin' Ted". Sanders got his own nickname: "Crazy Bernie."

Instead, many Democrats are taking the line that the election is about more than Trump. In a recent swing through Charleston, South Carolina, Senator Cory Booker made little mention of the President — except to urge voters to look beyond him.

Senator Kamala Harris has avoided using Trump's name even when she's clearly talking about him, instead decrying "powerful voices that are sowing hate and division among us" or declaring, "I will not conduct foreign policy by tweet".

That makes Sanders' approach stand out. His statement that "we must defeat Donald Trump" garnered sustained applause during a speech at the Iowa Fairgrounds, suggesting that it's a line many Democrats — at least Sanders' crowd — want to hear.

And steering too clear of the President carries other risks. If some Republicans regretted getting into a war of words with Trump in 2016, others concluded that it had been a mistake to ignore him for much of the campaign, on the theory that he couldn't win.

Sanders' language reflects his message that he's best positioned to defeat Trump, as well as an effort to assume the mantle of a frontrunner who is looking ahead to the general election.

"A central question for Democratic primary and caucus-goers is, who is the best candidate to take on and beat Donald Trump?" said Jeff Weaver, a senior Sanders adviser. "And I think Bernie Sanders provides a powerful contrast to Trump."

But Democratic pollster Celinda Lake said Sanders' attacks risk alienating some voters. Sanders has "a 25 per cent base that is fired up by this, that sent him US$6 million ($8.8m) in his first day", Lake said. "But is his job just to hang on to that base, or is his job to expand that base? I think others are going to find this too harsh — too much negative and not enough about the alternatives about what the Democrats provide."

Cheryl Ridgeway, a 61-year-old Iowa City resident who is undecided but leaning toward Senator Elizabeth Warren, said she doesn't like candidates "fighting back and forth" with Trump, but she can see the need for someone who can engage him.

"I want someone who appeals to our better angels, our better nature, but sometimes you've got to fight back," Ridgeway said.

Anti-Trump zingers set Sanders apart from the other candidates, and they excited the crowds that endured cold, rainy weather to see Sanders in his recent Iowa swing.

"Donald Trump is the elephant in the room. There's no avoiding it," said Jeff Shotwell, 67, who knocked on doors for Sanders in 2016 and attended one of Sanders' rallies.

Shortly after Sanders announced he was seeking the Democratic nomination, one supporter tried to turn the nickname bestowed by Trump, the 45th President, into a rallying cry. "Listen, I don't care when 45 calls him Crazy Bernie, because he is just a little crazy," said Black Lives Matter activist and writer Shaun King, who helped introduce Sanders at his kickoff event in Brooklyn this month. "And I think all of us got a little crazy in us, if you know what I mean."