Worried over what they described as President Trump’s erratic response to North Korea’s behavior, 64 Democratic legislators urged him on Tuesday to talk directly to the North Koreans — and warned that he would need congressional approval for any pre-emptive military strike.

“Few decisions are more needing of debate than a move to launch attacks, or declare war, on a nuclear-armed state such as North Korea,” read a letter signed by the lawmakers, led by Representative John Conyers Jr. of Michigan, the last Democrat in Congress to have served in the 1950-53 Korean War. “In such a volatile region, an inconsistent or unpredictable policy runs the risk of unimaginable conflict.”

The letter was sent against the backdrop of divergent signals from the Trump administration in recent weeks regarding North Korea’s nuclear and missile tests and its increasingly strident threats to use the “nuclear sword of justice” against the United States.

Mr. Trump has said that under the right conditions he would be willing to meet with North Korea’s 33-year-old leader, Kim Jong-un, who is less than half the president’s age. Mr. Trump also has expressed impatience with Mr. Kim and hinted at the possibility of a pre-emptive American strike on North Korea.