Former New York Mayor Rudy Giuliani stirred up controversy Sunday when he joined Mayor Michael Bloomberg on the campaign trail, warning an audience of orthodox Jewish voters that a vote for Bloomberg’s opponent in the mayoral race, African-American candidate William Thompson, would make the city comparable to modern-day Detroit.

“Detroit went from a great city with lots of good-paying jobs to a city that’s basically holding on for dear life,” Giuliani told the Boro Park Jewish Community at a rally for Bloomberg. He warned that with Thompson as mayor crime would escalate to levels it hasn’t seen since the early 90s, telling the crowd, “you know exactly what I’m talking about.”

Guiliani’s negative reference to Detroit and crime didn’t sit well with Fred Siegel, a historian and Giuliani biographer, who found the former mayor’s comments “indefensible” and racially divisive. Seigel told The New York Observer that Giuliani was using Thompson’s race against him. “If this isn’t a rude, racial invocation, then you don’t mention Detroit. The problem there is unsustainable pension costs,” Seigel said.