President Trump meets with North Korean defectors in the Oval Office on Feb. 2. (Reuters)

A group of Democratic senators is warning President Trump that he lacks the “legal authority” to carry out a preemptive strike on North Korea, amid questions over whether the White House is considering a risky “bloody nose” attack.

In a letter to be sent to Trump on Monday, the 18 senators said they are “deeply concerned about the potential consequences of a preemptive military strike on North Korea and the risks of miscalculation and retaliation.” They emphasized that it is an “enormous gamble” to believe that such an action, even if it were modest in scope, would not provoke an escalation from dictator Kim Jong Un.

“Moreover, without congressional authority, a preventative or preemptive U.S. military strike would lack either a constitutional basis or legal authority,” the senators wrote in the letter organized by Sen. Martin Heinrich (D-N.M.), a member of the Senate Armed Services Committee.

Congressional aides said the letter was prompted by the circumstances surrounding the sudden derailment of the White House’s original choice for ambassador to South Korea, a post that has remained vacant since Trump took office.

The Washington Post reported last week that Trump’s original choice, Victor D. Cha, was no longer expected to be nominated after more than six months of vetting. In December, the administration had sent Cha’s name to Seoul and received quick approval from the South Korean government, a formal process called “agrément” that typically is the final step before a candidate is nominated to the Senate.

The senators called his derailment “disturbing” and expressed  “serious concerns” over Trump’s handling of the post.

“We ask that you provide a clear reasoning and justification for his removal from consideration,” the senators wrote. The others who signed the letter are: Benjamin L. Cardin (Md.), Jack Reed (R.I.), Tim Kaine (Va.), Patty Murray (Wash.), Christopher A. Coons (Del.), Elizabeth Warren (Mass.), Edward J. Markey (Mass.), Brian Schatz (Hawaii), Jeff Merkley (Ore.), Bernie Sanders (I-Vt.), Jeanne Shaheen (N.H.), Catherine Cortez Matso (Nev.), Kamala D. Harris (Calif.), Mazie Hirono (Hawaii), Sheldon Whitehouse (R.I.), Chris Murphy (Conn.), Chris Van Hollen (Md.).

Administration officials said a red flag was raised in Cha’s background check, but they declined to offer details. Cha’s associates rejected the notion that a vetting issue scuttled the nomination, suggesting instead that he was a victim of the fallout over a policy dispute.

Cha had privately expressed concerns to National Security Council officials over their consideration of a limited strike on the North aimed at sending a message without sparking a wider war — a risky concept known as a “bloody nose” strategy. In an op-ed in The Washington Post shortly after the news broke, Cha made public his view that such a strike would be a huge risk to American troops and civilians living in South Korea and Japan.

“To be clear: The president would be putting at risk an American population the size of a medium-size U.S. city — Pittsburgh, say, or Cincinnati — on the assumption that a crazy and undeterrable dictator will be rationally cowed by a demonstration of U.S. kinetic power,” Cha wrote.

White House officials have rejected the notion that they are seriously considering a preemptive strike, pointing to their efforts to increase economic sanctions and diplomatic isolation of Pyongyang. Several administration officials said they have never heard the term “blood nose” strategy used in their deliberations over North Korea. Trump met Friday with a group of North Korean defectors to shine a light on the human rights abuses of the Kim regime.

As he has before, Trump criticized past administrations for not acting more forcefully on North Korea, though he did not specify whether he meant shutting down the nation’s nuclear weapons program or dealing with human rights violations, or both.

President Trump said on Jan. 30 that his administration is putting "maximum pressure" on North Korea, and pledged "American resolve" to honor Otto Warmbier's memory. (The Washington Post)

“Many administrations should have acted on this a long time ago when we weren’t in this kind of position,” the president said. “It could have been done 12 years ago, it could have been done 20 years ago. . . . We have no road left. We’ll see what happens. We’ll get through the Olympics and maybe something good will come out of the Olympics.”

The United States and South Korea agreed to suspend joint military exercises on the peninsula until after the two-week Winter Games, which begin Feb. 9 in PyeongChang. The North has agreed to send a delegation of athletes and officials and they will join the South during the traditional march into the opening ceremony. But experts said such displays of harmony are unlikely to last.

“Tensions on the Korean Peninsula are as high as they’ve ever been,” the Democratic senators wrote. “While we must always be ready to respond with decisive action to a North Korean provocation, it would be extremely irresponsible to instigate military conflict prior to exhausting every diplomatic option.”