President Donald Trump, left, North Korean leader Kim Jong Un, center, and South Korean President Moon Jae-in, right, walk together Sunday at the border village of Panmunjom in Demilitarized Zone, South Korea. Credit: Susan Walsh | AP

Hours after Donald Trump became the first sitting U.S. president to step into North Korea, 2020 Democratic candidates criticized his meeting with Kim Jong Un as the latest example of the White House’s governance by tweet, saying a handshake could not be compared to lasting progress between the two nations.

On Sunday, Trump said he and Kim had agreed to “work out some details” around negotiations over North Korea’s nuclear program. Trump has insisted that his relationship with Kim has grown stronger, even after a breakdown of their second summit in Hanoi four months ago.

“Nobody knows how things turn out, but certainly this was a great day,” Trump said after the talks with Kim. “This was a very legendary, very historic day.”

But Democratic president hopefuls dubbed the meeting “a photo op” and said it was far too soon to tell whether the talks will prompt meaningful political change. Summits with foreign leaders typically require advance staff work and preparations, they argued — not haphazard meetings with few concrete terms set in advance. Trump initially promoted his offer to meet with Kim at the border in a tweet while at the Group of 20 summit in Japan on Saturday morning.

“We’ve seen a history where Trump announces a summit and nothing really comes of it,” Sen. Amy Klobuchar, D-Minnesota, said on CNN’s “State of the Union.” “It’s not as easy as just going and bringing a hot dish over the fence to the dictator next door.”

Julian Castro, a former Housing and Urban Development secretary, said that North Korea hasn’t kept its promise from last year’s summit in Singapore to produce a detailed account of its nuclear weapons stockpile.

“I’m not quite sure why this president is so bent on elevating the profile of a dictator,” Castro told CNN.

“It’s all symbolism, it’s not substance,” he added.

Like his fellow candidates, Sen. Bernie Sanders, I-Vermont, said there was nothing inherently wrong with Trump sitting down with Kim, and that he would even like to see Trump meet with the leaders of Saudi Arabia and Iran. But once he’s at the table, Trump needs to push for more specifics, Sanders said.

“What’s going to happen tomorrow or the next day?” Sanders said on “This Week.” “We need to move forward diplomatically.”

Former congressman Beto O’Rourke, D-Texas, also criticized Trump’s handling of North Korea, saying on “Face the Nation” that Kim’s dictatorship hasn’t removed any of its nuclear weapons “or their attempt to deliver them to the U.S.” O’Rourke said that as president, he would “continue diplomacy contingent on progress.”

“Despite three years of almost bizarre foreign policy from this president, this country is no safer when it comes to North Korea,” O’Rourke said.

In a statement, former Vice President Joe Biden attacked Trump for “coddling” Kim “at the expense of American national security and interests.”

“His conduct reinforces that we urgently need a president who can restore our standing in the world, heal relationships with key allies Trump has alienated, and deliver real change for the American people,” Biden said in the statement.