Addressing an audience of mostly Labour Party activists in a community center in Cumbernauld, a town north of Glasgow, Mr. Miliband said, “From the head, from the heart, from the soul,” Scots should vote no.

“Don’t choose an irreversible separation,” he said. “Choose to stay together.”

The visits drew a mocking response from Alex Salmond, the leader of the Scottish government and of the pro-independence campaign, as he sought to maintain what has been widely depicted as momentum in the prelude to the vote. “No one believes their panicked pledges — it is a phony timetable for measly powers,” said Mr. Salmond, referring to promises of greater powers for Scotland within Britain, and a schedule to put them quickly into effect if Scots vote no.

The day of campaigning highlighted the problems facing opponents of separation, who have been accused of complacency and of running a negative campaign. Until now, Mr. Cameron has kept a relatively low profile, perhaps calculating that he is more likely to antagonize than charm Scottish voters.

Image Those in favor of Scottish independence campaign in Edinburgh on Wednesday. Credit... Jeff J Mitchell/Getty Images

His Conservative Party holds just one of the 59 Scottish seats in the British Parliament. As a wealthy Englishman who attended Britain’s most exclusive school, Eton College, Mr. Cameron is regarded by many Scots as out of touch. On Wednesday, Mr. Miliband declined to share a platform with the prime minister, telling reporters that he did not think doing so would help the “no” campaign.

“As the people of Scotland put pen to ballot paper next week,” Mr. Cameron wrote in an essay published on Wednesday in The Daily Mail, “you will be writing the U.K.’s future in indelible ink. It’s a momentous decision: There will be no going back.”

Supporters of independence say it would allow them to build a more socially inclusive nation, a message that seems to have been welcomed by many in Scotland, which tends to tilt more to the left than England does.