Union and city officials are sparring in advance of tough test score news that arrives at a pivotal moment for Mayor Bloomberg’s education legacy.

Scores due out on Wednesday reflect students’ performance on the first tests tied to the new Common Core standards, which aim to get students solving complex problems and thinking critically. State officials have long warned that the new tests would produce lower scores, which they say will more accurately reflect students’ skills, and in April, teachers and students reported that the tests were indeed challenging.

After the state sent a letter to principals on Friday confirming that the scores would be ”significantly lower” than in the past, the United Federation of Teachers argued — as it has before — that the news will undermine Bloomberg’s claims of education progress.

Chancellor Dennis Walcott called the union’s criticism “despicable” and “really sad” during a conference call with reporters on Sunday. “What they’re trying to do is politicize something that shouldn’t be politicized at all,” he said.

Instead, Walcott emphasized that the scores should be seen as a baseline against which to measure future improvement. Walcott and Shael Polakow-Suransky, the department’s chief academic officer, said they would not be comparing this year’s test scores to scores from past years.

“You can’t compare these directly because they’re not just slightly different tests, they’re dramatically different tests,” Polakow-Suransky said. “It’s going to be difficult to make close comparisons with old state exams.”

Without the handy comparison of year-to-year test score changes, which the Bloomberg administration has cited as evidence of success and to justify changes such as school closures, the city is grappling with how to chart academic change over time.

Polakow-Suransky said the National Assessment of Education Progress could still show whether city students have progressed. The NAEP test, which is administered to a sample of students by the National Center for Education Statistics and has been considered the only reliable metric for measuring progress in reading, math, and science across states, is not connected to the state’s old or new standards, although state officials have said one goal in adopting the Common Core was to bring the state’s standards in line with NAEP’s.

In February, a NAEP report found that test scores of students in New York State have not risen as quickly as scores in other large states. On the latest NAEP scores, from 2011, New York was one of only two states to post significant score declines since the previous test administration, while New York City’s scores were flat.

But because NAEP data can’t be used to assess individual schools or students, the department will also measure student growth by comparing students who scored at a similar level on last year’s exam, Polakow-Suransky said. Those comparisons will factor into school report cards this year, whereas test scores won’t factor into teacher evaluations until next year.

“But I want to be clear … no one is being punished because the test got harder,” he said.

Polakow-Suransky added that the department would also look at New York City’s scores compared to the rest of the state, especially other large cities that he said had not necessarily given teachers the same training and preparation for the new standards. In recent years, the Bloomberg administration has cited city students’ performance relative to their counterparts across the state as evidence that the city’s education policies are working.

Those policies are the ones that union officials have criticized as placing too much value on test scores — a criticism that UFT President Michael Mulgrew reiterated on Sunday.

“This week we are going to see that a decade of test prep — rather than real learning — has left New York City school children far behind where they should be,” Mulgrew said. “That is not ‘politicizing’ the issue. That’s reality.”

The union’s characterization of test prep happening in classrooms across the city is false, according to Polakow-Suransky.

“Anyone who spends time in classrooms knows that isn’t true,” said the chief academic officer, who said he sees “interesting, rich, creative and challenging assignments” in the vast majority of classrooms. Still, he said, he is not seeing enough complex assignments and critical thinking in city schools.

The city will receive the full set of state test scores today. Department officials said Sunday that they had seen preliminary data already but could not comment on it. Principals also have early data, but they are not permitted to share it, the officials said.

Correction: An earlier version of this story mischaracterized how test scores will be used in teacher evaluations. The test scores will be used for next year’s evaluations.