By Hilary Russ

(Reuters) - New Jersey Senate President Stephen Sweeney, the state's top elected Democrat, on Wednesday asked state and federal prosecutors to investigate alleged threats by a labor union that he said amounted to bribery and attempts to corrupt public officials.

The New Jersey Education Association (NJEA) threatened to withhold campaign contributions until Sweeney and other lawmakers passed a proposed constitutional amendment that would boost the state's public pensions, Sweeney said.

"These threats clearly cross the line from lobbying to attempted bribery and conspiracy," he said. The allegation was contained in two identical letters, released by Sweeney's spokesman, to both U.S. Attorney for New Jersey Paul Fishman and state Attorney General Christopher Porrino.

In a statement, NJEA President Wendell Steinhauer said the union "vehemently rejects" Sweeney's assertion "that any of our leaders or staff have done anything illegal or unethical."

He said that the NJEA "has been very clear that the pension amendment is our members' number one priority."

"Staff and leaders informed senators that Senate President Sweeney has failed to meet his explicit commitment" to advance the legislation, he said.

The ballot measure at issue would force the state to make its full annual contribution to its pension funds. Voters would then be able to approve or reject the constitutional amendment in November - if legislators first pass it.

The measure has been delayed in part because lawmakers said they would not advance it until they passed a separate, stalled effort to raise the state's gasoline tax in order to fund state transportation projects.

"NJEA's support for politicians is not an entitlement," Steinhauer said. "NJEA has simply informed legislators and party officials that we are withholding support that we are under no obligation to give."

A spokesman for Fishman said they had not received the letter. A Porrino spokesman said his office had received the letter and is reviewing it.

Sweeney is a long-time ironworker union member and presumed 2017 Democratic gubernatorial candidate, but his relationship with labor has not always been cozy. He alienated some when he worked with Republican Governor Chris Christie to reform the state's public pension system in 2011.

"No one is under obligation to give anybody a check," said Ben Dworkin, a political scientist at New Jersey's Rider University, who said the allegations raise legal questions as much as a political ones.

"The denial of a check is neither here nor there," he said. "It is simply, you used to have a friend in politics, now you don't."

(Reporting by Hilary Russ in New York; Editing by Jonathan Oatis)