WASHINGTON — U.S. lawmakers are offering legislation to limit President Donald Trump’s ability to launch a nuclear first strike after he heckled North Korea’s leader about the comparative size of his “nuclear button.”

Democratic Sen. Ed Markey, of Massachusetts, and Rep. Ted Lieu, of California, have sponsored legislation that would require the president to receive congressional approval before initiating a first-use nuclear strike from the United States.

The two took to Twitter to rally support for their legislation after Trump bragged in a tweet Tuesday evening that he had a “much bigger” button than North Korean leader Kim Jong Un.

[Read: Here’s what it takes to actually launch a nuclear strike]

“No one person should have the power to decide when the U.S. will be the first to use nuclear weapons,” Markey’s tweet reads in part. Retweet, if you agree, he asked; and as of Wednesday morning, more than 3,800 had.

In a hallway interview Wednesday afternoon, Senate’s No. 2 Republican John Cornyn, of Texas, expressed discomfort with the inflammatory talk on both sides. He stressed the need for a diplomatic solution and hailed the efforts of Secretary of State Rex Tillerson to engage China.

“That’s a very serious issue, and I don’t know how anybody’s interests are served by escalating that rhetoric,” Cornyn said.

A handful of congressional Democrats had earlier in the day taken to Twitter to decry Trump’s saber rattling, in somewhat stronger terms.

House Armed Services Committee member Ro Khanna, D-Calif., tweeted the Markey-Lieu legislation urgently needs to pass this Monday, the day the House returns.

Illinois Sen. Tammy Duckworth, a U.S. Army veteran who lost both her legs in the Iraq war, dissed Trump as “Cadet Bone Spurs” — a reference to his draft deferment during the Vietnam War — and urged him to worry more about mass U.S. military casualties in such a conflict.

Ohio Rep. Tim Ryan, a member of the House Appropriations Defense Subcommittee called Trump’s tweet “reckless beyond words.”

Trump mocked that assertion, writing: “Will someone from his depleted and food starved regime please inform him that I too have a Nuclear Button, but it is a much bigger & more powerful one than his, and my Button works!”

But despite ratcheting up the tension, Trump doesn’t really have a nuclear button.

The process for launching a nuclear strike is secret and complex and involves the use of a nuclear “football,” which is carried by a rotating group of military officers everywhere the president goes and is equipped with communication tools and a book with prepared war plans.

If the president were to order a strike, he would identify himself to military officials at the Pentagon with codes unique to him. Those codes are recorded on a card known as the “biscuit” that is carried by the president at all times. He would then transmit the launch order to the Pentagon and U.S. Strategic Command.

Earlier on Tuesday, Trump sounded open to the possibility of an inter-Korean dialogue after Kim made a rare overture toward South Korea in an address. But Trump’s ambassador to the United Nations insisted talks would not be meaningful unless the North was getting rid of its nuclear weapons.

In response to Kim’s overture, South Korea on Tuesday offered high-level talks on Jan. 9 at the shared border village of Panmunjom to discuss Olympic cooperation and how to improve overall ties. The South said Wednesday that North Korea’s state-run radio station announced the North would reopen a cross-border communication channel.

If there are talks, they would be the first formal dialogue between the Koreas since December 2015. Relations have plunged as the North has accelerated its nuclear and ballistic missile development that now poses a direct threat to America, South Korea’s crucial ally.

The U.S. administration, however, voiced suspicions that Kim was seeking to drive a wedge between Seoul and Washington. Pyongyang could view a closer relationship with Seoul as a way for reducing its growing international isolation and relief from sanctions that are starting to bite the North’s meager economy.

“We won’t take any of the talks seriously if they don’t do something to ban all nuclear weapons in North Korea,” U.S. Ambassador Nikki Haley told reporters at the United Nations. “We consider this to be a very reckless regime. We don’t think we need a Band-Aid, and we don’t think we need to smile and take a picture.”

The top Democrat on the Senate Foreign Relations Committee, Ben Cardin, said Trump’s tweet was “not befitting the president” and embarrassing on the world stage. He welcomed possible talks between Pyongyang and Seoul and said the U.S. must work with China toward a resolution.

“That’s their neighbor on the Korean peninsula, and it’s good to have talks between North and South,” Cardin said. “What the US needs to do is a diplomatic surge to resolve the crisis.”

The Associated Press contributed to this report.

Joe Gould was the senior Pentagon reporter for Defense News, covering the intersection of national security policy, politics and the defense industry. He had previously served as Congress reporter.

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