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(CNN) —  

Bernie Sanders turned back the clock four years on Donald Trump, hitting him for his first “birther” crusade – against President Barack Obama.

“As I mentioned to you earlier, my father was born in Poland. Like Obama, I am the son of an immigrant father. Nobody has ever asked me for my birth certificate. Maybe it’s because of the color of my skin being different than Obama’s,” Sanders said Monday to cheers from the students at Hoover High School in Des Moines, Iowa.

Facing a threat from Ted Cruz in Iowa, Trump has opened a line of attack questioning the Texas senator’s qualification for the White House because he was born in Canada. Cruz is considered a natural-born citizen, and legal experts consider him qualified for the White House, because his mother was born in Delaware, though the issue has not yet been tested in federal courts.

Four years ago, Trump was stoking the flames of the first birther movement – conservatives who questioned the validity of Obama’s birth certificate, his religion and his qualification to be president.

“He doesn’t have a birth certificate. He may have one, but there’s something on that, maybe religion, maybe it says he is a Muslim,” Trump told Fox News in 2011. “I don’t know. Maybe he doesn’t want that.”

Obama later produced his longform birth certificate, from Hawaii, and tore into Trump at the White House Correspondent’s Dinner in 2011.

“No one is happier – no one is prouder – to put this birth certificate matter to rest than The Donald,” Obama said. “And that’s because he can finally get back to focusing on the issues that matter: Like, did we fake the moon landing? What really happened in Roswell? And where are Biggie and Tupac?”

Sanders was answering a question from a Hoover High student who asked him what he thought of immigrants. He used the question to tear into Trump, who his aides have argued he is best positioned to take on in a general election matchup.