Candidates for the Democratic presidential nomination are speaking out against Donald Trump’s venture into North Korea, the first time a sitting US president has stepped into the country.The Republican president met with the North Korean leader in the Demilitarized Zone (DMZ) that divides North and South Korea on Sunday, in a meeting that appeared to have been planned on a whim earlier in the week. Joining many critics who called the journey a publicity stunt, nominees for the top Democratic spot voiced negative opinions of the weekend meeting.“Our President shouldn’t be squandering American influence on photo ops and exchanging love letters with a ruthless dictator,” wrote senator Elizabeth Warren on Twitter on Sunday morning. “Instead, we should be dealing with North Korea through principled diplomacy that promotes US security, defends our allies, and upholds human rights.”Senator Kamala Harris also used the platform to voice concern, tweeting “This President should take the North Korean nuclear threat and its crimes against humanity seriously. This is not a photo-op. Our security and our values are at stake.”In an interview on ABC, Senator Bernie Sanders said he had “no problem” with the president meeting with Kim Jong-un but that he didn’t want a meeting to “simply be a photo opportunity.”“What’s going to happen tomorrow and the next day?” senator Sanders continued. “He has weakened the State Department.”Candidate Julian Castro also said he did not condemn meeting with adversaries, but did express concerns Mr Trump was raising the profile of a dictator by meeting with the North Korean leader three times with nothing to show for it."He's doing it backward," Mr Castro said.A spokesman for former vice president Joe Biden said the president was "coddling" dictators at the expense of U.S. national security.Senator Amy Klobuchar, also running for the candidacy, compared the move to a moment between neighbours."It is not as easy as just going and bringing a hot dish over the fence to the dictator next door," she said.Additional reporting by Reuters