"These four freshmen are defining what these longtime politicians have to respond to. It's amazing to me, I mean Joe Biden's been in politics since 1973 and he is now taking his cues from a freshman Democrat, this 29-year-old to talk about what he wants to be for president," the former Trump administration official said on "The Story with Martha MacCallum."
"The agenda is being driven by these four freshmen members of Congress and it is an agenda that is going further and further towards socialism, embracing all of these things that in order for a Democratic candidate for president to get ahead they not only have to embrace but they have to one up."
Trump's tweets Sunday -- and comments since -- were aimed at progressive House Democrats Reps. Alexandria Ocasio-Cortez, D-N.Y., Rashida Tlaib, D-Mich., Ilhan Omar, D-MInn., and Ayanna Pressley, D-Mass.
Spicer said to MacCallum that the Democratic presidential candidates who adopt the views of "the squad" will have a hard time explaining their positions to Americans in the general election.
"The reality is is that when you have the front runners of the Democratic Party having to embrace and figure out how to one-up this... run to the progressive socialist left they're going to have a big problem when it comes to explaining themselves to the American public and in general," Spicer said.
Spicer notes how "extreme" the Democratic primary base had become.
"The people who are making up the constituency that will determine the primary and caucus-goers for the Democratic Party aren't in that lane. They're in the crazy left-wing shoulder," Spicer said.
Fox News' Nick Givas contributed to this report.