The word went out on Saturday morning, at 3.04am, from a laptop computer in Barack Obama's headquarters in a Chicago office building to about two million BlackBerrys and Twitters, that the Democratic vice-presidential nominee would be Joe Biden. It is one of the peculiarities of American politics that while vast numbers of people - more than 36 million in the Democratic primaries and caucuses this year - take part in the selection of the nominees for president, it is taken as an axiom that the vice-presidential nomination is selected by just one man.

In contrast, the British system gives prime ministers great powers, but not the power to designate successors. So we have the spectacle of Barack Obama, in his fourth year in the Senate, choosing as his running-mate Joe Biden, who is in his 36th year in the Senate. And we have the spectacle of Mr Biden, who said last year when he was running for president that he would not accept the vice-presidential nomination, now accepting it gladly. From a man he said then was not ready to be president.

The conventional wisdom is that the office of vice-president is “most insignificant” (John Adams) and “not worth a pitcher of warm spit” (John Nance Garner, bowdlerised). This was once true but is no longer. Americans took nearly 200 years to figure out what to do with a job whose occupant inherits the presidency in case of vacancy but which the Constitution assigns no significant duties in the meantime - but they finally did. The political parties have stopped nominating political nobodies who happened to come from a different region or different wing of the party to “balance the ticket” (the Obama-Biden ticket may be “balanced”, but Mr Biden is not a nobody). They no longer nominate rich men as vice-president in the hope that they will finance the campaign (that accounts for the Democrats' nomination in 1904 of an 80-year-old West Virginia industrialist named Henry Gassaway Davis).

And presidents no longer give vice-presidents nothing to do (Harry Truman in his 82 days as vice-president met Franklin Roosevelt only twice and the latter told him nothing about the atomic bomb). That changed when Walter Mondale, offered the number two slot by Jimmy Carter, demanded full access to the President and to top secret material, offices for top staffers in the West Wing and some real duties. President Carter delivered, and each president since has followed his example. American voters know the old jokes about the vice-presidency, but they have got the sense that vice-presidents matter.

In the hours after the nomination was announced, Democratic spinners faithfully recited their talking points. Mr Biden brings to the Democratic ticket and would bring to the White House some things Mr Obama does not: experience on foreign policy (he chairs the Senate Foreign Affairs Committee), white working-class roots (which he has been known to exaggerate but are still real), years of contact with ordinary people in a state so small voters expect to chat regularly with their senators. Republicans are noting that he talks fluently, and too much. The Delaware senator conquered a stutter as a teen and at age 29 won a job that allowed him to talk as long as he wanted. It is not surprising that he has committed numerous gaffes over the years, including plagiarising a Neil Kinnock speech, which are now being gleefully circulated by mass e-mail with links to YouTube.

Mr Obama has been on the offensive during most of this campaign, against Hillary Clinton and against George W.Bush. On this choice he was playing defence. The targeted demographic group - whites with working-class roots - voted heavily for Hillary Clinton in the primaries and has not rallied heavily to Mr Obama. It is concentrated in states, such as Pennsylvania and Michigan, which went Democratic in 2000 and 2004 and which Mr Obama evidently is worried about. At the same time the Obama campaign has dropped, temporarily it says, television advertising in Virginia, North Carolina, Georgia and Alaska, states that went for George W.Bush but where rising numbers of young upscale voters have been enthused by Obama's message of “hope and change”. A 65-year-old man who is in his 36th year in the Senate seems an odd choice to deliver that message. It's possible that the Democrats are retreating from ground on which they hoped to advance.

But military metaphors may not be apposite. Professor John Pitney, of Claremont-McKenna College, observes that this is the first key party ticket in a century in which neither nominee has either military or executive experience. The last one (orator William Jennings Bryan and John W.Kern, who was twice beaten for governor in Indiana) was nominated by the Democrats, in 1908, in Denver. It lost to a ticket led by a former secretary of war and governor-general of the Philippines. Democrats here in Denver are putting on a happy face about the Biden nomination, and he surely helps the ticket. But they are not entirely sure that their two senators will beat the former navy pilot and his choice for veep.

Michael Barone is the principal author of The Almanac of American Politics

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