FASLANE, Scotland » Britain’s aging nuclear-armed submarines, the force that keeps the nation in the first tier of global powers, have been the target of spirited if largely ineffectual protests by anti-war activists for more than three decades here in the fleet’s home port.

But now Scotland’s growing political importance has made the future of the nuclear arsenal an issue in Britain’s general election campaign, intensifying debate over whether the country can afford its nuclear deterrent, a quarter-century after the end of the Cold War.

That question is being asked because of the surging popularity of the Scottish National Party, which stands to the left of the opposition Labour Party and has promised to rid Scotland of nuclear weapons, putting it at odds with both Labour and the Conservative-led government.

Despite losing its bid last year for Scotland to win independence from Britain, the Scottish National Party has gained strength in the polls and could tilt the balance of power if, as happened in the last national election in 2010, neither Labour nor the Conservatives win an outright majority in Parliament in the voting May 7.

Should Labour win the opportunity to form a government and turn to the Scottish National Party for support – a prospect analysts say is very real despite a Labour promise not to enter a formal coalition with the party – the question of abandoning the Trident missile system, moving the fleet from Scotland, or at least delaying an expensive modernization program would be on the table.

Malcolm Chalmers, director of research at the Royal United Services Institute in London, said that the Scottish National Party will try to make the fleet an issue of principle, and use the nuclear debate to advance its effort to make Scotland independent.

"I don’t see a scenario in which Britain will give up Trident because of the Scottish National Party, but I do see them making it awkward for the nuclear force with regard to renewing the submarine fleet, and of using the issue as a way of demonstrating their logic – that you have to have independence if you want to remove nuclear weapons from Scotland," he said.

Chalmers added, "There are a lot of people among the political classes and senior military figures who would like a third way in which Britain could remain a nuclear power but spend a lot less money on it."

The problem, he said, is that "it’s much harder to work out what that would be."

Britain’s strategy is to have a nuclear-armed submarine permanently at sea as a deterrent against attack. Experts say that reducing the number of submarines from the current four would undermine that stance because spare submarines are needed for maintenance reasons, or in the event of collisions. A delay in a planned upgrade of the fleet might produce few savings because of the additional costs of patching up submarines that will be 35 years old by the late 2020s.

But the politics of Scotland is now crucial to the future of Britain’s nuclear deterrent. Angus Robertson, the Scottish National Party’s defense spokesman, said that Faslane is a symbol of how Scottish views on Trident have been "totally ignored" and that "repeated British governments have made decisions over the heads of people in Scotland."

Robertson said the proximity of Faslane to Glasgow, less than 40 miles away, makes Scotland’s largest population center a target for any enemy attack, fueling hostility to nuclear weapons. "There is overwhelming public opposition," he said, citing stances taken by activists, churches and labor unions, among others.

It was in the 1980s that camps of protesters against nuclear weapons sprang up around Britain, the best known at Greenham Common in Berkshire. After the end of the Cold War, they mostly disappeared, but not at Faslane. Even on a cold and bleak recent winter day, a handful of volunteers huddled by wood stoves inside camping vehicles and wooden structures across the highway from the Faslane base, which is encircled by a fence topped with razor wire.

Others are more concerned about the cost of modernizing the fleet, estimated by the government at 25 billion pounds (nearly $40 billion) – and by Robertson at perhaps four times that amount – and whether Britain can afford to remain in the nuclear club.

Those on the left would like this money spent on social programs. Some members of the defense establishment believe it would be better spent on conventional forces. David Ramsbotham, a former British army general and current member of the House of Lords, said he is "very worried that we’ve got an unbalanced defense budget" with too much tied up in "expensive things like aircraft carriers, Typhoon aircraft, submarines and, of course, Trident."

He described the logic of Britain’s nuclear deterrent as "out of date thinking," and said "the decision on Trident must be based on the analysis of our position in the world relative to it."

With Britain likely to fall below its NATO military spending target of 2 percent of gross domestic product, the debate seems sure to intensify.

A decision is due next year on whether to modernize the fleet of Vanguard submarines that carry the Trident system. If the Conservatives win the election in May, Trident’s future seems assured. Speaking in Parliament in January, the defense secretary, Michael Fallon, said that the naval base is "truly Britain’s peace camp" and that the country "faces the threat of nuclear blackmail from rogue states."

The Scottish National Party has ruled out cooperation with the Conservatives.

Asked about a possible deal with Labour, Robertson said eliminating the Trident program was one of three issues that "will be key to the SNP after the election." (The others are more expansionary economic policies and greater powers for politicians in Scotland.)

A minority Labour government might still be able to push through a modernization of the fleet with the support of the Conservatives. Ed Miliband, the Labour leader, has ruled out a full coalition with the Scottish National Party, while keeping open the option of a looser arrangement. But if such a deal came about, the Scottish National Party might refuse to agree to government spending plans that included the renewal of Trident, perhaps forcing an indefinite delay.

Not all Scots want to kill off Trident. In Helensburgh, a few miles from Faslane, Michael Curley, owner of the Buffet Shop, a delicatessen, said the demise of the nuclear fleet would "devastate" the town. Faslane, he said, is a "recession-free" employer immune from economic downturns.

But Scottish opponents include Feargal Dalton, a former British naval officer who served at Faslane and who, as a submariner, fired an unarmed Trident missile in an exercise.

Dalton now works as a teacher and represents the Scottish National Party on the Glasgow City Council, but still proudly wears a submariner’s badge.

He believes that the rationale for Trident is rooted less in military logic than in the desire of London politicians for Britain to be seen as "a world player."

"Scots," he added, "don’t see things like that."

That is certainly true at what organizers claim is the world’s oldest peace camp, a collection of brightly painted caravans, rickety wooden structures and banners opposite the well-guarded naval base. Chris Higgins arrived here last year and, a few weeks later, put on his wet suit, got into a canoe and began paddling quietly toward the sensitive military installation before finally setting off an alarm that brought armed, amphibious police.

"It is scary enough when the searchlight hits you," Higgins said, adding that, despite being arrested and fined, he would do it all again, even at the risk of drowning "or getting shot."

Stephen Castle, New York Times