LONDON — Americans fear assassination; Brits fear character assassination.

Like most baby-boomers, I remember 1968 — but not for students and hippies, flower power and anti-Vietnam War protests. No, the two things I remember most clearly are: Russian tanks snuffing out the Prague spring, and the assassinations of Martin Luther King and Bobby Kennedy.

At the time, the death of Kennedy loomed much larger than that of King; Today, my children study the civil rights movement in detail but have barely heard of the Kennedys.

It was said that Colin Powell refused to run for president because his wife feared an attempt would be made to assassinate him. It must have taken courage for Barack Obama to run the gauntlet — but he let his guard down with his comment about small town America clinging to religion and guns. No wonder Mr. Obama likes gun control: presidential candidates are among the few who think that the Second Amendment right of Americans to bear arms is at best inconvenient, at worst terrifying — but nobody made them apply for the job.

American politicians fear assassination; British politicians fear character assassination. The first British prime minister to die at the hands of an assassin’s bullet, Spencer Perceval in 1812, was also the last.

Even though in 1984 Margaret Thatcher was fortunate to survive the Brighton Bomb and at least one plot to kill Tony Blair was foiled, Downing Street only recently has adopted the permanent state of vigilance that surrounds the White House.

By now the British are catching up fast in the security stakes. Driving past Mr. Blair’s private residence last week, I counted at least four police officers stationed at possible entry points — and he is only a former prime minister.

Within living memory, however, prime ministers routinely ignored security and took incredible risks. I remember standing on the roof garden of Shell-Mex House in the Strand, now Penguin’s London headquarters but that of the Air Ministry in wartime, and marvelling at Winston Churchill’s chutzpah. For this is where the then prime minister often used to stand during the London Blitz, after dining next door at the Savoy Grill, to watch the bombs falling on London — at great risk to himself.

According to the diaries of Quintin Hogg, later Lord Hailsham, in 1964 the then prime minister, Sir Alec Douglas-Home, was almost kidnapped: not by terrorists, but by a group of students.

It happened like this: Douglas-Home (pronounced “Hume”) was staying with his friends Lord and Lady Tweedsmuir near Aberdeen in Scotland. Some Leftists students had followed the prime minister, intending to kidnap him for a few hours as a stunt before releasing him.

His bodyguard had been left behind in town and Sir Alec was alone in the house. When he answered the door, the deputation of students told him they were going to kidnap him. Douglas-Home coolly replied: “I suppose you realise if you do, the Conservatives will win the election by 200 or 300.”

He decided to play for time. The students gave him 10 minutes to pack. “After that they were offered and accepted beer.” That did the trick. The Tweedsmuirs came home and the kidnap was over. Not only were the police not told, but the bodyguard swore his boss to secrecy “as his job would have been in peril” — the bodyguard’s, that is, not Sir Alec’s.

The sheer nonchalance with which Douglas-Home handled his predicament commands respect. He told the story many years later.

The present incumbent at Downing Street is also a Scot, but it is hard to imagine two more contrasting types. Douglas-Home had renounced his hereditary earldom in order to be prime minister; Gordon Brown does not think his predecessor should have had a title to renounce. While Sir Alec, typically, hushed up the case to save his bodyguard’s job, Mr. Brown is notorious for bullying his staff.

For so long, the British public, apparently willing to take Mr. Brown’s reputation as a steward of the economy on trust, suddenly has changed its mind. His poll ratings have plummeted faster than Neville Chamberlain’s in 1940, just before he resigned to make way for Churchill. The Tories have a spring in their step, with everything to play for.

Last week Mr. Brown was in America. His trip was upstaged by the Pope’s; even after two visits, most Americans still don’t know who he is. Mr. Brown hoped to look and sound statesmanlike. Back home, however, the rats were deserting his statesmanship.

His plan to raise taxes on the poor has aroused even the normally supine Labor Party. Last year the party allowed Mr. Brown to succeed Tony Blair unopposed. This week he has had to buy off a revolt in the House of Commons, led by Frank Field, a lean and hungry Cassius whom Mr. Brown has thwarted for more than a decade.

It now looks quite possible that Mr. Brown could be overthrown by a coup before the next election. The assassins are gathering at the capitol. All they need now is a Brutus.

Mr. Johnson is the editor of Standpoint.