The same is not true of Scottish independence. Analysts say Mr. Johnson’s government is not prepared for the wall of pressure it will face if the Scottish National Party wins a majority. The last time the party achieved that, in 2011, Britain’s then-prime minister, David Cameron, yielded to demands for a referendum. In 2014, Scots voted against leaving Britain by 55 percent to 44 percent.

Polls now put the split at roughly 50-50, after a stretch in which the pro-independence vote was solidly above 50 percent. Analysts attribute the slight softening of support to both the vaccine rollout, which showed the merits of staying in the union, as well as an ugly political dispute within Scottish nationalist ranks.

Mr. Johnson holds a trump card of sorts. To be legally binding, an independence referendum would almost certainly have to gain the assent of the British government, so the prime minister can simply say no and hope the problem goes away. But that strategy can work for only so long before becoming untenable.

“I don’t see any way in the world that Boris Johnson turns around the day after the election and says, ‘OK, you can have a referendum,’” said Nicola McEwen, a professor of politics at the University of Edinburgh.

And yet the calls could only grow. “If they manage to peel off a single-party majority,” she said, “it does put pressure on the U.K. to answer the question, ‘If a democratic vote isn’t a mandate for independence, then what is?’”

View Source