ST. LOUIS — One of the most important assembly lines here is getting pretty short.

In late 2016, the final F/A-18 Super Hornet is due to roll out of Boeing’s fighter-jet plant north of Lambert-St. Louis International Airport. When it does, thousands of jobs at Boeing and hundreds of suppliers will leave with it, and another big chunk of the region’s manufacturing base will slip away.

Nearly three years might be an eternity in some industries, but it’s a blink of an eye in big-ticket defense manufacturing. Indeed, parts for that final F/A-18 are already starting to be manufactured. That means the aerospace giant is fast nearing crunchtime in its bid to keep the Super Hornet line alive.

“This is a critical year for us,” said Mike Gibbons, Boeing’s vice president in charge of the program.

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For Boeing, extending production of the Super Hornet and its high-tech cousin, the E/A-18 Growler, means keeping a foothold in a global fighter-jet market that’s being taken over by Lockheed Martin’s F-35, and keeping a skilled workforce and supply chain that can help it win the military’s next big jet.

For St. Louis, it means thousands of good jobs and hundreds of millions of dollars a year in the local economy. For taxpayers and the military, though, more Super Hornets may simply be more than they need.

Boeing is coming to the end of its last multiyear contract to build Hornets and Growlers, the workhorse of the Navy’s air fleet. That deal plus a contract with the Royal Australian Air Force, along with a recent decision to slow production from four planes per month to three, will stretch the line into late 2016.

Any orders beyond that must come now, in the 2015 federal budget that will start to wind through Washington next month. Pentagon budget-writers are drafting their final recommendations for military spending, to be released by the Obama administration March 4.

A Navy spokeswoman declined to comment on F/A-18 spending before that date. But Loren Thompson, a defense analyst with the Lexington Institute, a Washington-area think tank, said it’s unlikely defense officials will seek the about $2 billion it would take to buy another year’s worth of Super Hornets.

Instead, he predicts, the Navy will put its limited resources toward the F-35C, its version of the Joint Strike Fighter, and start upgrading the Navy fleet now.

“That’s the bottom line,” Thompson said. “The Navy cannot afford to both keep buying Super Hornets and to develop a new, more stealthy alternative. It has opted to go with the new plane.”

But that won’t be certain until March — and Boeing is still making the case for the Super Hornet. Last year it unveiled new upgrades designed to boost the plane’s stealth capabilities, radar and range.

And Gibbons touts his jet, with its $52 million “fly-away” price tag, as a proven, cost-effective, “no drama” alternative to the F-35C, which didn’t even start flying until 2010 and hasn’t yet begun aircraft-carrier testing. That argument, along with delays and cost overruns on the Joint Strike Fighter program, helped Boeing win its last multiyear contract in 2010.

Despite improvements to the F-35 program, there’s “a chance” that argument could prevail again, and win the Super Hornet one more reprieve, said Todd Harrison, who studies the defense budget at the Center for Strategic and Budgetary Assessments in Washington.

“2015 was supposed to be the year the Navy went with the F-35C,” he said. “But we’re seeing some indications that they’re at least considering not making that decision.”

Boeing, too, says it likes what it’s hearing from the Navy and remains hopeful about the March budget. But it’s also preparing to take its case to Congress to press for the Super Hornet even if the Pentagon makes no request.

“We’ll be calling our congressmen and our regional support networks,” Gibbons said.