The Metropolitan Transportation Authority's tentative deal reached Thursday with New York City bus and subway workers could undercut LIRR unions' leverage in a showdown with the MTA negotiators next week and push railroad employees closer to a strike, union officials said.

One Long Island Rail Road union official, speaking on condition of anonymity, said the deal with Transport Workers Union Local 100 for raises of 8 percent over five years is "terrible" for LIRR laborers because it diminishes their bargaining power in seeking bigger raises at a Monday meeting with a mediation board empaneled by President Barack Obama and increases the likelihood of a strike.

"Now, I've completely lost faith in the process," the official said.

The LIRR labor source said he expects railroad union leaders to stick to recommendations of an earlier Obama board -- a 17 percent, six-year contract -- as their best offer. But, the slimmer Transport Workers Union contract could set a new, lower benchmark for federal lawmakers if they are forced to legislate an LIRR contract to end a strike, which could come in July, the source said.

MTA board member Mitchell Pally agreed that the deal with TWU, representing more than 34,000 New York City subway and bus employees, now becomes "the measuring rod" for all other labor agreements.

"You can't give to one what you didn't give to the other," Pally said about the deal's terms.

MTA chairman Thomas Prendergast said the agency would continue abiding by the federally mandated bargaining process with the LIRR unions, but noted TWU contracts historically establish "a pattern of how the other [unions] will follow."

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TWU spokesman James Gannon declined to comment Thursday on how the agreement could affect the railroad unions, but said LIRR contracts are typically influenced more by other commuter, passenger and freight railroad labor agreements, and less by those of subway and bus employees.

In reaching the TWU deal, Prendergast said the agency backed off its demands for a three-year wage freeze and settled for a deal "consistent with our financial plan" and that would not affect fares. The contract includes new employee health care cost contributions and new health benefits.

"We got to a point within the last 48 hours where we could get there, and we got there," Prendergast said.

Thomas Creegan, a TWU union officer, agreed that the tentative contract could be a raw deal for LIRR workers, as he said it is for bus and subway employees.

"As much as everyone else might celebrate that this is behind them, this ain't a victory. Am I supposed to do a handstand because I got a 1 percent raise ?" Creegan said. "I really hope it doesn't hurt the Long Island Rail Road."

The agreement comes after the TWU vowed to support LIRR unions "in every way possible" -- even suggesting that it would refuse to help the MTA execute its contingency plan in the event of an LIRR strike. But the LIRR union source said the TWU appeared to turn its back on the railroad workers at a crucial time in their negotiations.

"We were supporting the railroad unions out of solidarity," TWU spokesman Gannon said. "We both went years without contracts."

The TWU deal, which still has to be approved by the board of the MTA and TWU and by union members, comes after two years of negotiations, during which the unions balked at the MTA's "three net zeros" wage freeze demands.

A TWU source noted that, while the total raises may be smaller, the LIRR unions could argue that the TWU deal is comparable with what they are seeking because the railroad unions have gone longer without a contract. LIRR unions haven't had a contract since 2010. In 2010 and 2011, TWU workers got raises totaling 7 percent.

Gov. Andrew M. Cuomo announced the deal in his Manhattan offices while flanked by Prendergast and TWU Local 100 President John Samuelson. Cuomo said the contract was fair both to the MTA and to its "great public servants."

Regardless of the specific terms of the agreement, Kevin Law, president of the Long Island Association -- a business group -- said the progress made between the MTA and its unions "may help to avoid a strike that would cause serious economic disruption on Long Island."