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It was a shrewd tactic, said Robert Bruno, professor of labor and employment relations at the University of Illinois at Chicago.

“This union figured out they couldn’t assume the public would be on their side, so they went out and actively engaged in getting parent support,” Mr. Bruno said. “They worked like the devil to get it.”

But, some reform advocates said, public opinion could swing against the union relatively soon if the dispute seems to carry on with no resolution in sight.

Juan Jose Gonzalez is the Chicago director for the education advocacy group Stand for Children, which has hundreds of parent volunteers and was instrumental in pushing legislative reforms in Illinois. He says parents “are all over the map” in terms of their support for teachers or the school district.

“Within a day or two, all parents are going to turn their ire toward the strike,” Mr. Gonzalez said. “As parents see what the district offers and see the teachers not counterpropose, they will become increasingly frustrated with the grandstanding.”

Already, there are some parents who don’t understand why teachers would not readily accept a contract offering a 16 percent raise over four years — far more than most American employers are giving in the aftermath of the Great Recession.

Rodney Espiritu, a stay-at-home dad whose 4-year-old son just started preschool, said the low test scores he’s read about suggest teachers don’t have “much of a foot to stand on.”

In a telephone poll conducted Monday by the Chicago Sun-Times, nearly half of people surveyed said they supported the teachers union, compared with 39 percent who oppose the strike. Almost three-quarters of those polled regarded Mr. Emanuel’s efforts to resolve the dispute as average, below average or poor. The poll of 500 registered voters had a margin of error of plus or minus 3.8 percentage points.

Associated Press writers Sophia Tareen, Michael Tarm and Jason Keyser contributed to this report.