On the night of his primary election victory, city comptroller candidate John Liu stood in the city’s teacher union headquarters and thanked the United Federation of Teachers for delivering his win. In the mayoral race, by contrast, the UFT chose to sit on the sidelines and not endorse the Democratic candidate, as it has historically done.

How much of a difference has the UFT’s decision to sit out the race made for comptroller Bill Thompson’s campaign? The answer likely rests on the continuum between not much and not at all, election observers said today.

Those who argue that a UFT endorsement would have helped Thompson, if only modestly, point to the UFT’s powerful voter turnout machine. In an election predicted to see few voters, the ability to mobilize teachers and parents could be a deciding factor in who wins tomorrow.

A spokesman for the union, Dick Riley, estimated that union volunteers had made about 200,000 calls and distributed 50,000 pieces of campaign literature this year on behalf of endorsed candidates in citywide, borough and city council elections. The union also sends out robocalls urging its members to vote for candidates and its president, Michael Mulgrew, made appearances with candidates at press conferences.

To the Thompson campaign, which was been criticized for lacking discipline, an army of UFT volunteers with supplies on hand would have been a welcome sight. But Thompson may need more help than that, observers said.

“If they had endorsed Thompson it would have been a big plus, but it wouldn’t have been enough” said James Vlasto, a former communications director for public advocate Betsy Gotbaum.

Vlasto, who has worked on 30 campaigns in New York City, said the UFT’s support would not have raised Thompson’s poll numbers significantly, as the campaign’s flaws were too pronounced to be solved with one endorsement.

The Thompson campaign “had some unions, they had vehicles that could spread the word for him, [but] they missed out,” Vlasto said. “They spent all their time and money saying eight years is enough, and that’s a fine slogan, but there are other issues.”

Other observers said no amount of phone-banking or literature-distributing by the UFT would have made a difference for the Thompson campaign, as it was already mid-October when a chapter leader at a delegate assembly meeting offered a resolution to endorse Thompson. The motion was postponed and the union never returned to it.

“Without significant efforts by a union following a late endorsement in a race, the impact of the endorsement can be negligible,” said Benjamin Kallos, the director of policy and research for Mark Green’s campaign for public advocate.

This is not the first time the UFT has decided wait out a mayoral election, especially during contract negotiations. Following the end of contract negotiations in 2005, the UFT chose not to endorse Bloomberg or his Democratic challenger Fernando Ferrer. The UFT was also neutral in 1993 and 1997.

“It’s not uncommon that unions will not take a position when they’re sitting at the bargaining table,” said District Council 37 Local 372 president Veronica Montgomery-Costa. DC 37 endorsed Thompson over the summer and Montgomery-Costa spoke from the campaign’s headquarters where union volunteers were working.

“If they don’t pick the right candidate it could have a devastating impact on negotiations,” she said.

Vlasto said the UFT’s decision to keep the last two mayoral campaigns at arm’s length had caused it to cede political power to the the Working Families Party, an idea Riley disputed. “As to our clout, both De Blasio and Liu made it a point to address our delegate assembly after their endorsements,” Riley wrote in an email. ”

“One presumes that Liu had some reason for having his victory party after the runoff here at the UFT and Cy Vance (endorsed by the UFT but not the WFP) singled Mulgrew out at his primary night celebration to thank him for the UFT’s contribution to his victory.”

A spokesman for the Thompson campaign did not return requests for comment.