The Chicago Teachers Union threw its formidable endorsement behind Toni Preckwinkle in the crowded race for mayor, one day after the former school teacher embraced the union’s education agenda.

Well aware of the power of the CTU’s endorsement, Susana Mendoza’s campaign was ready with a counter-punch: National Farm Workers Association co-founder Dolores Huerta.

The dueling endorsements came Wednesday as the race for mayor heats up.

CTU’s backing comes as Preckwinkle is calling for a “fully-elected” school board, a freeze on new charter schools and public school closings for the four years until that board is seated and “real progressive revenue” to bolster neighborhood schools and student support services.

She wants tax-increment-financing surpluses funneled exclusively to Chicago Public Schools, and she wants existing charter schools re-evaluated, using the same rigid standards used to assess public schools.

The only CTU policy Preckwinkle hasn’t embraced — but hasn’t ruled out, either — is the so-called “LaSalle Street tax” on Chicago’s financial exchanges.

Now that Preckwinkle has signed on to the CTU’s education agenda, it was no surprise that the union’s House of Delegates ratified the CTU executive board’s decision to endorse.

Leaders of the CTU, SEIU Healthcare and SEIU Local 73 have scheduled a joint news conference for Thursday where they will discuss their endorsements of Preckwinkle.

Four years ago, Preckwinkle was the CTU’s first choice to challenge Mayor Rahm Emanuel. When Preckwinkle took a pass, then CTU-President Karen Lewis stepped up to fill the void, only to be diagnosed with brain cancer.

The CTU’s third-choice, Jesus “Chuy” Garcia, managed to force Emanuel into a run-off, but fell short.

Huerta joined Cesar Chavez in 1962 in co-founding the National Farm Workers Association and has been fighting for immigrants and working families ever since, endorsed Mendoza Wednesday in a digital ad released by the Mendoza campaign.

“The work of my life has been standing up for workers, immigrants, women, human rights, and civil rights,” Huerta was quoted as saying in the ad.

“Susana Mendoza is the next generation to carry on this fight. She’s a battle-tested fighter who champions the little guy and is an experienced, committed, and hardworking visionary. Sí, se puede con Susana Mendoza.”

Mendoza, who is on a long-planned vacation, could not be reached for comment.

In a campaign press release, Mendoza describes Huerta as a “personal hero and role model throughout my life” and said she was “honored to have her endorsement.”

“Dolores is a tireless fighter who has spent her life standing up for women, worker, and immigrant rights. As mayor, I will be a steadfast advocate for the social justice causes we both believe in and never back down when our values are under threat,” Mendoza was quoted as saying.

“With the support of Dolores and grassroots activists like her across Chicago, I know we can tackle the big challenges we face, put our neighborhoods first, and lift up every community across our city.”

In order to do that, Mendoza must first survive a strong petition challenge from Preckwinkle that threatens to knock her off the ballot.

Preckwinkle election lawyer Keri-Lyn Krafthefer has challenged the validity of 16,746 of the 25,660 signatures that Mendoza submitted a week ago.

That would drop Mendoza well below the legal requirement of 12,500 signatures.

“A lot of different candidates have identified the same flaws as we have,” she said.

Mendoza’s supporters have accused Preckwinkle and other mayoral opponents of ganging up on her to keep her out of the race.

As for the battle for union endorsements, they’re only as good as the money and manpower they produce for the candidate.

SEIU Local 1, the United Food and Commercial Workers Union Local 881 are certain to translate into money and manpower for Preckwinkle.

So will the CTU — just as it donated millions to and made thousands of foot soldiers available to work the precincts for Garcia four years ago.

But Huerta’s endorsement may be little more than one highly-regarded labor leader’s opinion.

SEIU Local 1 is part of a group of unions that now owns the Chicago Sun-Times.