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By Bernd Debusmann Jr
BBC News, Washington
President Joe Biden has blasted criticism of Nato by his likely 2024 election challenger, Donald Trump, as "dumb", "shameful" and "un-American".
The Democrat assailed Mr Trump for saying at the weekend that as president he would not defend Nato members unless they met their defence spending quotas.
Mr Biden said the remarks underscored the urgency of passing a $95bn (£75bn) foreign aid package for US allies.
The bill just passed the Senate, but faces political headwinds in the House.
At the White House on Tuesday, Mr Biden said a failure to pass the package - which includes $60bn for Ukraine - would be "playing into Putin's hands".
He said the stakes have risen because of Mr Trump's "dangerous" remarks on Saturday.
"No other president in history has ever bowed down to a Russian dictator," Mr Biden said.
"Let me say this as clearly as I can. I never will. For God's sake. It's dumb. It's shameful. It's dangerous. It's un-American."
At a rally on Saturday in South Carolina, Mr Trump, a Republican, criticised "delinquent" payments by Nato members.
He recounted a past conversation with the head of "a big country" about a potential attack by Russia.
Mr Trump said the official had asked if the US would defend a Nato member that had not met its financial obligations.
"I said: 'You didn't pay? You're delinquent?'" Mr Trump told the crowd. "'No I would not protect you, in fact I would encourage them to do whatever they want. You gotta pay.'"
Mr Biden said on Tuesday that his predecessor was treating the military alliance like a protection racket.
"As long as I'm president," the president said, "if Putin attacks a Nato ally, the United States will defend every inch of Nato territory."