If Russia used nuclear weapons to try and salvage its faltering invasion of Ukraine, as recently threatened by warring leader Vladimir Putin, it would amount to “an attack … on NATO itself,” Sen. Lindsey Graham said Thursday.

The South Carolina Republican told reporters that the US and its Western allies would have to devise an “overwhelming” response that would be “catastrophic” to Russia should that doomsday scenario play out.

“From my point of view, the use of a nuclear weapon by Russia in Ukraine would be an attack on NATO itself,” Graham said.

“The radiation would not be confined to Ukraine, you would be irradiating parts of Europe that are under the NATO banner,” he added.

“So, if that day ever comes — and I hope and pray it doesn’t — that should be considered by NATO writ large and the United States as an attack by Russia on NATO itself.”

Russia invaded Ukraine in February, ostensibly to squash its overtures to the 28-member alliance.

Graham made the remarks at a joint news conference with Sen. Richard Blumenthal (D-Conn.) introducing legislation that would block US assistance to any country that recognized Russia’s looming annexation of four eastern Ukraine regions.

The bipartisan duo had previously proposed a measure that would designate Russia as a state sponsor of terrorism, a label that the Pentagon and White House have resisted.

Putin was expected to formally begin annexing the occupied territories Friday following a sham referendum in which Russian troops went door-to-door compelling local residents to cast ballots in support of Moscow rule.

The hostile takeover came a week after Putin said 300,000 conscripts would be sent to fight in Ukraine, and threatened to “use all the means at our disposal” to protect “the integrity of our country.”

Graham said the US and its allies should not take the threat lightly.

“As Putin becomes more provocative we have to stand up, not stand down,” he said.

“The last thing you can do when you’re bullied is to give in to the bully.”

With Post wires