When the news spread that Joe Biden was going to be Barack Obama’s running mate, Republicans reacted with glee. The loquacious senator for Delaware, they were sure, was a target-rich opportunity for ridicule. E-mails are already circulating among party activists about the bumper stickers planned by conservative attack groups reading “Obama Bin Biden”.

“Republicans are going to be ready, willing and eager to run against him,” said a party consultant who received the Obama Bin Biden e-mail. “There’s a smirk on our faces because there is plenty to work with.”

Once the laughter fades, there is an underlying seriousness and decency about Biden, 65, that will make him a formidable vice-presidential candidate. He has age, charm, working-class roots, attack dog instincts and foreign policy expertise on his side.

“Voters are smart enough to forgive the genuine flaws of genuine people,” David Brooks, the conservative columnist, observed in The New York Times. “And over the long haul, Biden provides what Obama needs.”

Biden was on the fast track to greatness when he was elected to the Senate in 1972 at 29, even though he could not take office until he was 30. But he became a figure of fun for plagiarising Neil Kinnock in a speech while running for president in 1988. The then Labour leader had asked: “Why am I the first Kinnock in a thousand generations to be able to get to university?” Biden echoed: “Why is it that Joe Biden is the first in his family . . . ?”

After a chastening gap of 20 years, Biden ran for president again this year, but lacked the money and dazzle to compete with Obama and Hillary Clinton. He promptly revived his foot-in-mouth tendencies by describing Obama as “the first mainstream African-American who is articulate, bright and clean”. He also claimed that Indians were emigrating in droves to his tiny home state: “You cannot go to a 7-Eleven or a Dunkin’ Donuts unless you have a slight Indian accent. I’m not joking.”

However, he also has the experience and wit to serve as an attack dog – the traditional job of a running mate – against John McCain, the Republican nominee. Biden is chairman of the Senate foreign relations committee and knows as many world leaders as McCain. He flew to Georgia last week to discuss the Russian crisis with President Mikhail Saakashvili. His son, Beau Biden, a captain in the Delaware National Guard, is to deploy to Iraq as a military lawyer in October.

Obama is looking to Biden to strip McCain of the aura of heroism resulting from his time as a prisoner of war in Vietnam. It has been noted that there is comic potential in the way McCain and his team have been overdoing the references to his incarceration whenever they are put on the spot, but it is not a challenge that Obama, 47, can take on.

Biden ruthlessly skewered Rudolph Giuliani, the former mayor of New York, during the Republican primaries with mockery. “There are only three things he mentions in a sentence: a noun, verb and 9/11,” he said.

Biden’s verbosity may be compensation for the stutter that he suffered from as boy. He was nicknamed Dash because he could never finish a sentence.

He was born into a Catholic family in Scranton, Pennsylvania, a working-class town in a critical swing state, and understands the “Joe Sixpack” voters who gave Obama the brush-off during the primary campaign. His father grew up playing polo and sailing yachts but, after his business failed, the family fortunes never recovered.

Within weeks of Biden’s election to the Senate in 1972 his wife Neilia and their 13-month-old daughter, Naomi, were killed when a drunken lorry driver crashed into their car. His two young sons, Beau and Hunter, were seriously injured and he was sworn in as senator at their hospital bedside.

Biden’s sister helped him with childcare while he commuted an hour and half by train from Washington to Delaware every day to be home with them. It is a habit he continues and, after more than 35 years in the Senate, this gives him some claim to retaining “outsider” status – an important qualification for Obama because of his message of “change”. He has a daughter by his second wife, Jill.

In 1988 he nearly died from two brain aneurysms, but he has not had health problems since. Biden’s age makes him ideal for the job in the mode of Dick Cheney, the incumbent. Because he is unlikely to run for the White House himself in eight years’ time, he can be trusted to give Obama disinterested advice. In the event of a tragedy, he would have the experience and trust to be president on day one.

Hillary Clinton hailed Biden yesterday as “an exceptionally strong, experienced leader and devoted public servant”.

During the primary campaign Biden said that Obama was “not yet ready” for the presidency, a Clintonesque criticism that is already coming back to haunt him in an advertisement issued yesterday by the McCain camp. They spliced his dismissive words about Obama with comments Biden made in 2005 saying he would be “honoured to run with or against McCain”.

Unlike Obama, Biden initially backed the war in Iraq, although he has prominently criticised its conduct. Once again, however, it is a trajectory that he shares with American voters.

The first hint that Biden really was Obama’s choice for vice-president came when he showed uncharacteristic discipline and stopped talking to the press last week.

Asked during a presidential debate whether he was too wordy to lead the free world, he gave a one word answer: “No.” It suggests he is not too old to change his spots.

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