TRENTON -- Senate President Stephen Sweeney rejected calling for a crucial vote Monday on a referendum asking voters to constitutionally guarantee state payments into the government worker pension fund, killing its chances of appearing on the November ballot and disappointing public labor unions.

The likelihood that Sweeney, once the prime champion of that amendment, would hold the vote has grown increasingly slim in recent weeks as the pension question became embroiled in an impasse over transportation funding.

Monday marked the deadline for the state Senate to vote to place the referendum on the fall ballot. The Assembly has already passed the measure.

If approved by voters, the amendment would have required the state make increasing contributions into the pension system, which is short about $43.8 billion, to reverse decades of underfunding. The state would have needed to drum up an additional $550 to $800 million a year to make the payments.

Sweeney (D-Gloucester) has maintained that the clash over the transportation funding cast doubt over the viability of the pension amendment.

"Without a resolution to the Transportation Trust Fund crisis -- and a full accounting of how much future tax cuts will cost -- it would have been too easy for opponents to argue that the state could not afford to pass the pension amendment," he said in a statement Monday afternoon. "The pension amendment would have been doomed to defeat, and that would have given carte blanche to current and future governors to slash pension payments."

Sweeney noted Monday that with a simple majority the Legislature can still put the referendum on next year's general election ballot at no loss to public workers because the state is already complying with the payments schedule.

But labor unions have made clear they won't tolerate another "broken promise," holding yet another protest outside the Statehouse calling on Sweeney to post the measure.

They also cautioned they would harness their votes, manpower and money come June, when voters are expected to have a crowded pool of Democratic primary candidates for governor, including Sweeney, to choose from.

"We're going to work diligently to talk with every one of (the New Jersey Education Association's) 200,000 members and make sure they know what went down here this summer," said Brian Rock, vice president of the East Orange Education Association. "June 2017 will be a day of reckoning, and June can't come too soon."

Wendell Steinhauer, president of the New Jersey Education Association, the state's largest public union, said it will "take all the energy that we had planned to use in the constitutional amendment and put it ... into finding the right people to serve in government that will do the right thing for the taxpayers of New Jersey."

The NJEA is the state's most powerful public-sector union, contributing millions of dollars to Democratic campaigns through its well-endowed political action committees.

The pension amendment was expected to spark an expensive summer fight between labor unions, who say the constitutional amendment is the only way to ensure the bill gets paid, and business groups that warned it would spur massive spending cuts or tax hikes.

Earlier this year, business organizations across the state formed a coalition to launch a campaign against the amendment and other Democratic-backed proposals they said would damage New Jersey's tax and business climate.

For his part, Gov. Chris Christie argued it would foist a $3 billion tax increase on the people of New Jersey.

In his February budget address, the governor said the amendment "places government workers ahead of every other citizen of the state in our state budget."

"Ahead of our students. Ahead of your hospitals. Ahead of the disabled. Ahead of our seniors. And the burden of the cost would be put on the backs of taxpayers," he said.

Sweeney has argued the state budget could absorb the increased pension costs with modest, 3 percent, growth in state revenues.

Samantha Marcus may be reached at smarcus@njadvancemedia.com. Follow her on Twitter @samanthamarcus. Find NJ.com Politics on Facebook.