"This is a watershed moment," the New Jersey Democrat said on CNN's "State of the Union." "The European Union, NATO, as well as the U.S., have to consider that this is dramatically different, and we need to give Ukrainian forces the resources to fight for themselves."
The White House has so far been hesitant to say explicitly that Russia is "invading" Ukraine or "declaring war." Earlier this week, President Obama characterized the latest military action by Russia as an extension of the fighting that has been going on between the two countries for weeks.
But Menendez, who is traveling currently in Kiev, Ukraine, did not mince words — and said the U.S. must now consider taking more action to curb Russia's encroachment.
"We have to look at sectoral sanctions," Menendez said. "In this case, weakness is a greater provocation for [Russian President Vladimir] Putin to act than strength."
Republicans have been quicker to classify Russia's recent attacks on a new region in Ukraine as an invasion, and House Intelligence Committee Chairman Mike Rogers echoed that party line Sunday on Fox News.
"It's hard to argue that Russian troops with Russian armor crossing the border into Ukraine doesn't fit the definition of an invasion," Rogers said. "I don't think they're there on a sightseeing tour."