Trump and Kim hailed their positive relationship on Sunday and Trump said negotiation teams would be "working to see whether or not they can do something" over the next few weeks, but Democrats questioned whether meaningful change could be achieved.
Former Vice President Joe Biden's campaign spokesman Andrew Bates released a statement Sunday criticizing Trump of "fawning" over Kim and joking with Russian President Vladimir Putin about U.S. election security and "getting rid of" journalists.
"President Trump's coddling of dictators at the expense of American national security and interests is one of the most dangerous ways he's diminishing us on the world stage and subverting our values as a nation," Bates said.
"Of course as a country we want this to work, we want to see denuclearization of the Korean Peninsula, a reduction in these missiles. But it is not as easy as just going and bringing a hot dish over the fence to the dictator next door," she said. "This is a ruthless dictator and when you go forward, you have to have clear focus and a clear mission and clear goals. And that has been our problem so far."
On the same program, former Housing and Urban Development Secretary Julian Castro said Trump has approached past meetings with Kim and other leaders "very haphazardly" and planned this meeting "at the last minute."
"The problem with that is that, to be effective, this usually goes the other way around," the candidate said. "There is a lot of staff work that goes into preparing a meeting like this, so that concrete terms are on the table and you can get something out of this meeting."
Castro also noted North Korea hadn't fulfilled a promise to produce an inventory of their weapons stockpile as agreed upon during a previous summit between the two leaders in Singapore.
"I'm not quite sure why this president is so bent on elevating the profile of a dictator like Kim Jong Un when Kim Jong Un has not lived up to his promise from the first summit," he said.
Appearing on ABC News' This Week, Sen. Bernie Sanders, I-Vt., said Trump's past negotiations have been compromised by "incredible inconsistencies" and described Sunday's meeting as a "photo opportunity."
"The whole world's media was attracted there," Sanders said. "What's going to happen tomorrow and the next day? He has weakened the State Department. If we're going to bring peace to this world we need a strong State Department."
"They have removed none of their nuclear weapons or their potential to deliver them to the United States and in fact, in contravention of the United Nations they have launched their missiles flouting the diplomacy that this president has attempted so far. So, we've added legitimacy to Kim Jong Un," he said.