Every so often, Washington politicians or strategists accidentally let their masks slip.

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Earlier in the campaign, Hillary Clinton dropped one of these revealing verbal bombshells when, on May 23, she opined that a good reason for her to stay in the race against Barack Obama was that, quite simply, he might get killed.

"We all remember Bobby Kennedy was assassinated in June in California," Clinton told the Sioux Falls South Dakota Argus-Leader newspaper.

The reaction was such that Clinton had to deliver a stream of explanations and apologies.

Unspeakable beliefs

In fact, the Obama campaign received so many threats of violence that the candidate received secret service bodyguard protection months earlier than is customary for American presidential candidates.

And now we have Charlie Black, John McCain's chief strategist, voicing another unspeakable yet widely held belief: A terror strike would be good for McCain.

"Certainly it would be a big advantage to him," Black told Fortune magazine.

He added that the assassination of Benazir Bhutto, while an "unfortunate event" was a really lucky break for McCain, giving him a boost in the crucial New Hampshire primary.

Bhutto's killing, Black believed, reminded voters of how dangerous a place the world is, making McCain's supposed foreign affairs and national security expertise a bigger campaign asset.

'Politics of fear'

As with Clinton's gaffe, Black's blunder engendered much scolding from pundits.

Calls for him to resign from the McCain team quickly followed. McCain distanced himself from the remarks, saying he "strenuously disagreed" with his top strategist.

Obama also brought out Richard Ben-Veniste, former member of the 9/11 commission, who told a conference call with reporters that Black's remarks "were so out of place that they call for some recalibration in the thinking and perhaps a greater adherence to principle here in staying away from the politics of fear".

Of course, Obama could argue that a new al-Qaeda-type attack on US soil would be the direct result of the ruinous policies of the Bush administration, particularly in Iraq and Afghanistan, which McCain has endorsed and promises he will continue.

Black's comment struck a nerve among many Democrats, who believe some sort of crisis abroad or an attack at home would in fact hurt Obama badly.

It is a sign of the more dreadful times we live in, I suppose.

Back when Bill Clinton was running for the White House, his aides fretted constantly about evidence of their boss's sexual activities coming to light -"bimbo eruptions", they called them.

It seems trivial compared to our darker fears now, doesn't it?

A painful precedent

For Democrats, there is a painful precedent for Black's doomsday scenario.

A tape by bin Laden emerged just before the 2004 US elections [EPA]

And Kerry aides were not the only ones saying so at the time.

McCain himself said of the bin Laden eruption: "I think it's very helpful to President Bush. It focuses America's attention on the war on terrorism. I'm not sure if it was intentional or not, but I think it does have an effect."

"Intentional?" Did McCain think maybe, just maybe, bin Laden may prefer Bush in the White House?

That perhaps al-Qaeda's numero uno was enjoying the spectacle of Bush shattering America's public image, delighting over the needless deaths of so many American troops, and relishing the thought of thousands of recruits freshly radicalised by the war in Iraq flocking to his banner?

Were bin Laden and Bush in cahoots? Was there a White House hotline - in a locked desk drawer in Karl Rove’s office, perhaps - that ran direct to a certain cave in Waziristan? 

All hyperbole aside, the fear that he will be at a disadvantage in the national security department is one reason Obama seems likely to pick a combat veteran, former military commander or longtime foreign policy expert - think Jim Webb, Sam Nunn, or Wesley Clark - to be his vice-president.