Other supporters of the program said Brown’s support for the GE engine does not represent a shift on his campaign promises, because Brown believes the engine, notwithstanding the Pentagon’s position, is critical to US national security. Many in Congress have argued that establishing competition for the engine contracts could hold down costs in the future.

By lending his support to the GE backup engine, Brown is mirroring a longstanding position of the late senator Edward M. Kennedy, whose seat Brown filled after a special election in January.

The Pentagon since 2006 has said that the primary engine for the F-35, built by Pratt & Whitney, in East Hartford, Conn., is sufficient, and that producing a backup model will siphon away money that could be spent on other defense priorities.

“If Scott Brown helps out GE he will be doing exactly the opposite of what he said he would do when he ran,’’ said Loren Thompson, a defense budget specialist at the Lexington Institute in Arlington, Va., which is supported by multiple defense firms, including Pratt & Whitney.

“This is yet another example of how ‘fiscally responsible’ lawmakers have a giant blind spot when it comes to defense spending in their districts,’’ said Laura Peterson, a senior national security analyst at Taxpayers for Common Sense, a nonpartisan budget watchdog group that monitors earmarks. “His support was clearly driven by parochial concerns rather than financial ones.’’

But spending critics say Brown is abandoning his personal pledge to battle pork barrel spending in Washington.

Brown, who landed a coveted spot on the influential Armed Services Committee, joins other Massachusetts politicians who in the past have successfully advocated for GE’s jet engine, which could generate several hundred jobs at GE’s engine plant in Lynn.

Brown “was disappointed that the administration’s budget cut funding for the Joint Strike Fighter alternate engine and will work with fellow members of the delegation to find a solution,’’ the statement said.

Brown’s office said in a statement to the Globe that the freshman senator believes “this project benefits both our military and our state.’’ It was Brown’s first public statement on how he will vote on military spending matters.

The Bay State Republican’s support for General Electric’s bid to build a backup engine for the F-35 Joint Strike Fighter puts the new senator in the middle of a confrontation over congressional earmarks with the Obama administration, which has threatened a presidential veto if Congress inserts funding for the engine for the fifth year in a row.

WASHINGTON — Senator Scott Brown says he will fight to fund a multibillion-dollar weapons program that could generate jobs in Massachusetts but that the Pentagon insists it does not need, sparking criticism that Brown is breaking his campaign vow to rein in wasteful spending.

Brown was unavailable for an interview yesterday because he was traveling overseas.

Brown has hammered at the federal government for needlessly spending taxpayers’ money. In announcing his Senate candidacy last September, he criticized his Democratic opponents for “pandering to the special interests, promising to support their pet project.’’

“That’s not the way I operate,’’ Brown said.

At a December press conference, Brown said that “spending is out of control,’’ and that “government keeps getting bigger and bigger.’’

And in January he called for a freeze in wages for federal workers “until a plan is devised for controlling government spending and debt.’’

His positions have attracted crucial political support. One ad campaign, paid for by the US Chamber of Commerce, included the tagline, “Scott Brown believes in fiscal responsibility.’’ And one of Brown’s political mentors, Senator John McCain of Arizona, a leading voice against GE’s F-35 backup engine who cosponsored unsuccessful amendments to kill it, has said he supported Brown because of his pledges to reduce needless spending.

“He epitomized what the American people were saying,’’ McCain said, introducing Brown to a crowd in Phoenix last month. “The American people were saying: ‘Stop. . . . We don’t want out-of-control spending in our nation’s capitol.’’’

The fight over the backup engine, which remains in the design phase and is opposed by Pratt & Whitney, promises to heat up again this year. GE and Pratt & Whitney have hired retired generals to lobby on their behalf, and GE has accused Pratt & Whitney of secretly funding a watchdog group that is highly critical of the GE engine. Both Pratt & Whitney and the group, Citizens Against Government Waste, declined to say whether they have a financial relationship.

Thus far, the GE engine has received $2.4 billion for development work, much of it never requested by the Department of Defense.

Pentagon officials estimated in February that it would require an additional $2.9 billion over the next six years to continue development. They also said they found no evidence to support assertions that giving the military two engines to choose from will drive down costs when the fighter aircraft fleet is built in large quantities.

Despite the negative reviews, the Massachusetts congressional delegation has been instrumental in keeping the program alive. Kennedy was one of its biggest champions. And Senator John F. Kerry, a Massachusetts Democrat, played a key role in securing nearly $500 million for the program last year, saying that it was critical to the local economy, and that having an alternate engine would foster much-needed competition in the Joint Strike Fighter program. The program is the largest such program in history, with 2,443 planes set to be built by Lockheed Martin for the Navy, Air Force, and Marine Corps.

Kerry said he plans to push again to secure, or “earmark,’’ funding for the project. “I’ve already been on the phone with the White House, [Office of Management and Budget], and the Pentagon,’’ he said.

Representative John Tierney, a Salem Democrat whose district includes the GE plant, has strongly defended the program.

GE spokesman Rick Kennedy said about 150 of the 2,500 jobs now working directly on the project are located in Lynn, with the rest at facilities in Indiana, Ohio, and elsewhere. If the engine enters production, he said, Lynn’s role would grow to at least 500 jobs, possibly as many as 1,000.

The debate, however, is likely to hinge on how the military handles major contracts, not job creation.

“Competition in defense contracting helps keep costs under control and realize savings over time,’’ Tierney said. “Additionally, the risks are too great to have only one supplier of engines.’’

He cited recent revelations that the Pratt & Whitney engine is experiencing cost increases and delays.

Other powerful lawmakers — including Representative Ike Skelton, a Missouri Democrat who chairs the House Armed Services Committee — also assert that the F-35 program is simply too important to rely on a single engine manufacturer.

“When 95 percent of the Department’s fighters will be F-35 variants by 2035, this is not a question of pork,’’ Skelton said at a recent congressional hearing. “It is a sincere concern for the success of the F-35 program and for the benefits of competition.’’

Yet the secretaries of defense in both the Bush and Obama administrations, and the military’s top generals, have insisted for years that they simply cannot afford to finance development of a second engine.

“We have reached a critical point in this debate where spending more money on a second engine for the [F-35] is unnecessary, wasteful, and simply diverts precious modernization funds from more pressing [military] priorities,’’ William J. Lynn, the deputy secretary of defense, told Congress in a February memo.

Bryan Bender can be reached at bender@globe.com.

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