At the start of the whole Center Right Meme, Jon Meacham wrote:

But history, as John Adams once said of facts, is a stubborn thing, and it tells us that Democratic presidents from FDR to JFK to LBJ to Carter to Clinton usually wind up moving farther right than they thought they ever would, or they pay for their continued liberalism at the polls. . . . The pattern has deep roots. FDR had a longish run (from 1933 to 1937), but he lost significant ground in the 1938 midterm elections . . .

You see Meacham says FDR's Dem Party "lost" in the 1938 election (of course, FDR did not lose, the Dems maintained huge majorities in the Congress) because of FDR's "liberal overreach." The thesis is that FDR's "court packing" scheme was "liberal overreach I suppose (though of course the 1937 Supreme Court term famously provided FDR with victories for his New Deal.) Or perhaps FDR's attempts to knock out some of the more conservative Dem members of Congress in favor of more progressive ones is what Meacham is talking about (though that does not make sense as FDR largely failed in this effort.) Paul Krugman provides a different thesis - that FDR stumbled in the 1938 election because he turned Center Right: