While Republicans rave about Sarah Palin and denounce the media coverage of her daughter’s pregnancy, a number of the party’s elders are in a state of high anxiety.

It’s not that they are worried about any specific political fallout from the pregnancy revelation. In fact, some suspect that the Governor of Alaska and her family will elicit sympathy from the American public when she speaks at the Republican convention tonight in her first big set-piece performance. It is more that they are increasingly concerned about what the sudden spate of stories from her past says about John McCain’s judgment.

Some Republicans are plainly upset that in an election campaign that Senator McCain himself has said turns on the central issue of national security, he has chosen someone as a potential successor in a crisis who, whatever her other talents, has no background in international affairs.

One former Bush Administration official described himself to me as “personally disgusted” by the selection, one that betrayed a desire by Mr McCain for short-term political gain at the expense of the national interest.

But the bigger worry among many Republicans here is not that they might win in November and Mrs Palin later prove to be ill-equipped to lead the nation should she have to, but that they might lose; that the cascading revelations about her will bring down the McCain campaign.

At issue are the judgment and attention shown by the McCain campaign in selecting her.

It has become axiomatic in the past 30 years that vice-presidential choices do not really make much difference to presidential elections. But that may be because presidential candidates have tended to go for caution, with well-known national political figures as their running-mates. When they have strayed from that formula, perils have generally followed.

Unlike presidential candidates, who have to endure a long primary campaign under an intense media spotlight, vice-presidential candidates emerge just a couple of months before election day. They are, therefore, much more vulnerable to the risk that something in their past or something they say may throw the presidential campaign off balance and off message.

Since 1976, every vice-presidential nominee from either party had served in the US Congress. Many had also held other significant national office: George Bush Sr had been head of the CIA; Dick Cheney was a former Defence Secretary.

In an ominous sign for Mr McCain, the only times that running-mates have done real damage have been when they were relatively unknown. In 1984, even though she was a member of the House of Representatives, Geraldine Ferraro was not a national figure and under the pressure of the campaign, revelations about her husband’s business connections did irreparable damage to Walter Mondale. Spiro Agnew, of Maryland, was the last state governor to be picked as a vice-presidential nominee. Though Richard Nixon won in 1968 and again in 1972, Vice-President Agnew was dogged by corruption allegations and forced to resign in 1973.

The most notorious selection of a hitherto unknown vice-presidential candidate was Senator Tom Eagleton, picked by the Democrat George McGovern in 1972. He was chosen after only the briefest of background checks, and within weeks he was forced to leave the ticket after disclosures about his past mental health.

Some Republicans see parallels between these three selections and Mr McCain’s decision to go with the unsung Mrs Palin. They are especially concerned that he seems to have picked her after having met her only briefly once and after what looks like,to say the least, a somewhat sketchy background check.

The McCain camp, and plenty of other Republicans, insist that the current media frenzy will blow over and that after the initial spate of revelations about her, there will be nothing in Mrs Palin’s background to do real harm.

But the anxiety — and alarm at Mr McCain’s high-stakes gamble — is palpable. As David Frum, a former speechwriter for President Bush, put it: “When someone takes the rent money and puts it on black at the roulette table, and it comes up black, we don’t say, “Wow! What a terrific piece of judgment.”

Times Recommends

• 'New York has lost its edge'
• Copenhagen will fail – and quite right too
• More than 17,000 new underwater species found