Democrats will not pick their presidential nominee for another year but Donald Trump and former vice president Joe Biden are acting like the 2020 contest is already a two-man race.
But it has been the president's recent focus on Mr Biden that has surprised his allies and critics, who believe the Republican president may be unintentionally aiding a man whose candidacy is barely a month old.
Mr Trump’s advisers have privately encouraged him to lay off Mr Biden. He has done the opposite, uttering more public insults than at any other Democrat over the month.
During a state visit to Japan on Monday, he agreed with North Korean leader Kim Jong-un's recent description of Mr Biden as a "low-IQ individual".
A Biden campaign aide on Tuesday called the comments “beneath the dignity of the office".
The campaign said it waited a day to respond out of respect for Memorial Day, a holiday that honours the service of military veterans.
“To be on foreign soil on Memorial Day and to side repeatedly with a murderous dictator against a fellow American and former vice president speaks for itself,” deputy campaign manager Kate Bedingfield said.
Democrats in key primary states report that the intensifying feud has strengthened Mr Biden’s argument that he is best positioned to defeat Mr Trump in 2020.
Regardless of the short-term political consequences, it is becoming increasingly clear that Mr Trump and his allies consider Mr Biden their most formidable opponent.
The assault could become more organised and intense should he maintain his position as favourite for Democrats endorsement in the coming months.
“Trump seems to be obsessed with Biden, which suggests that he’s afraid of him,” said Gary Pearce, a longtime Democratic operative in North Carolina.
Republican strategist Alice Stewart agreed: “Clearly, the reason Trump is going after Biden is because he’s perceived as the biggest threat.
“From Biden’s standpoint, you couldn’t ask for a better situation.”
Mr Trump’s attacks are not part of an organised strategy, insiders say.
They say Mr Trump is tweeting and the campaign and the Republican National Committee is working overtime to keep up
It is consistent with Mr Trump’s view of himself as his own political strategist, prioritising his gut feelings over the advice of aides.
In recent weeks, the president has attacked Mr Biden’s intelligence, his energy level, his history of unwanted touching, his record on criminal justice reform and his dedication to Pennsylvania.
Mr Trump told Pennsylvania voters during a rally last week that Mr Biden deserted them.
Mr Biden, vice president under Barack Obama, is a native of Scranton, Pennsylvania, but left the state as a child when his parents relocated for work.
“I’ve never forgotten where I came from," he tweeted. "My family did have to leave Pennsylvania when I was 10.
"We moved to Delaware where my Dad found a job that could provide for our family. Trump doesn’t understand the struggles working folks go through.”
But the president went further on Monday when he ridiculed Mr Biden on foreign soil.
“I don’t take sides as to who I’m in favour or who I’m not,” Mr Trump said in Japan, when asked if he would side with Mr Kim rather than Mr Biden.
“But I can tell you that Joe Biden was a disaster. Kim Jong-un made a statement that Joe Biden is a low-IQ individual.
"He probably is, based on his record. I think I agree with him on that.”
On Tuesday, Mr Trump said he “was actually sticking up” for Mr Biden in Japan.
He said that Mr Kim had referred to his rival as a “low-IQ idiot” and that he had “related the quote of Chairman Kim as a much softer ‘low-IQ individual.’
“Who could possibly be upset with that?” Mr Trump tweeted.
His insults against Mr Biden have been the subject of significant internal debate in the White House and his re-election campaign.
Several officials, including his senior adviser and son-in-law Jared Kushner, have expressed reservations that the president is helping Mr Biden, the insiders say.
Mr Trump may be highlighting Mr Biden’s vulnerabilities for progressive voters, especially when it comes to his history on criminal justice issues.
But some aides believe Mr Trump risks turning Mr Biden into a celebrity for Democratic voters.
Mr Biden’s blue-collar appeal in Michigan, Pennsylvania and Wisconsin poses the deepest threat to Mr Trump's path to 270 electoral votes.
Others in Mr Trump’s orbit are less worried and believe Mr Biden might be the easiest to beat of the Democratic field.
“I happen to believe Biden would be the weakest of the general election candidates because he’ll be carrying 47 years of baggage and will have many of same vulnerabilities as (Hillary) Clinton,” said Mr Trump’s 2016 senior communications adviser, Jason Miller.
Billionaire Republican donor Doug Deason, who sits on the finance committee for the most powerful pro-Trump super PAC, cheered the president's approach.
“Why is he going after Biden? Biden is leading in the polls. He can beat ’em up,” Mr Deason said.
“I think he’s the only real legitimate contestant in the field. And I think it would be a shame for Democrats to end their race to the left and nominate him.”
But few Biden supporters view the attention from the president as a bad thing.
Mr Biden’s campaign seized on Mr Trump’s attacks in a Tuesday fundraising email titled “Donald Trump is scared.”
“Over the last week, President Trump has repeatedly insulted Joe Biden, even going so far as to side with North Korean dictator Kim Jong-un," the email said.
"Why? It’s simple: Trump’s afraid he’s going to lose. Let’s use this opportunity to show Donald Trump he should be scared of our momentum.”