Trump, at Rally in Florida, Kicks Off His 2020 Re-election Bid

Mr. Trump needs “to make suburban voters ask themselves by going to the polls, ‘What am I more annoyed by, Trump’s or the Democrats’ beliefs?’” said Steve Deace, an influential conservative radio host in Iowa.

But that calculus gets muddled in a scenario in which former Vice President Joseph R. Biden Jr. emerges from his party’s nominating fight. Mr. Trump has been telling advisers that running against Mr. Biden would be a reprise of his 2016 race against Hillary Clinton, another more centrist candidate with a long track record who was anathema to the progressive wing of the Democratic Party.

Optimistic Democrats see danger ahead for the president.

“Trump begins the race in a perilous place,” said David Axelrod, a former top political adviser to Mr. Obama. “He is viewed unfavorably in the very Midwestern states that delivered him the White House, and it isn’t obvious where he would pick up states to replace them.”

Mr. Trump’s dreary polling numbers come despite a strong economy, which generally portends good things for an incumbent president. But Mr. Trump’s advisers have found, alarmingly, that voters do not credit him for it.

And Mr. Trump often steps on his list of accomplishments on jobs, tax cuts, deregulation and the appointment of conservative judges.

Still, his campaign aides feel confident of his re-election chances, mostly because of their dim view of the Democratic field. He is backed by a campaign operation that is sleeker and more sophisticated than the ragtag team he ran out of the 26th floor of Trump Tower in 2016. The campaign has invested millions of dollars in a digital strategy to harvest emails and phone numbers from potential supporters, and to advertise on sites like Facebook and YouTube, where his supporters can be found.

Nonetheless, Mr. Trump remains his own biggest asset and liability.

“This is a candidate, a president and personality who just throws out the script and improvises,” said Kevin Madden, a Republican political consultant. “He’ll probably operate within the stagecraft they provide him, but the message discipline you would expect from an incumbent campaign launching a re-election? It’s not going to look anything like that.”