John McCain marked his 72nd birthday yesterday by selecting the 44-year-old Governor of Alaska, Sarah Palin, as his running mate, gambling that her lack of experience and profile on the national, let alone international, stage will be offset by her appeal to women and conservative voters.

"When you get to know her, you are going to be as impressed as I am," Mr McCain told a boisterous rally in Dayton, Ohio, acknowledging his pick will be unfamiliar to most Americans. He called Mrs Palin a reformer and a Washington outsider happy to challenge special interests and the status quo.

In an interview last year with this reporter, Mrs Palin lamented the paucity of leadership in the Republican Party, saying it was weighed down with "good old rich white boys". She mentioned no names.

"I know it will demand the best that I have to give," Mrs Palin said yesterday. In an appealing personal touch, she revealed that she and her husband Todd Palin, who works for British Petroleum, were celebrating their 20th anniversary yesterday also. Once a teenage beauty queen in Wasilla, a railroad town north of Anchorage, Mrs Palin told supporters in Dayton that not long ago she was "just your average hockey mom".

Her story of political rung-climbing in a male-dominated state is compelling. She later served as Wasilla's first female mayor and then rose to become chairwoman of the state's Oil and Gas Commission where she oversaw the ousting of a fellow Republican exposed for corruption. She became the first female Governor of Alaska in 2006 and its youngest ever. Several qualities may appeal to the party base. She is a fervent supporter of new drilling to achieve oil independence, as Mr McCain is. She holds conservative social views. Evangelicals will salute the decision she made last year to bring a fifth pregnancy to full term even knowing her new child would have Down's syndrome.

In the interview, given at her Alaska home when rumours of her vice-presidential potential were first circulating, she made comments that may now come back to haunt her. Notably, she uncharitably compared the prospects of her own party in this election year with Democrats when they were fielding such candidates as Barack Obama and Hillary Clinton.

"That's diversity right there, isn't it? Wow! And who do the Republicans have? Good old rich white boys. I think that's another factor that has to be considered by Republicans, that in some way their candidates are a reflection of more politics as usual. Not to slam good old rich boys, but it sure wouldn't hurt for new energy and new perspective to be enveloped by the Republican Party," she said.

Mr McCain called her "someone who has fought against corruption polices and failed policies of the past... someone with experience who has shown great tenacity and skill in tackling tough problems. She has strong principles, a fighting sprit and deep compassion".

The Obama campaign wasted no time before pouncing. "Today, John McCain put the former mayor of a town of 9,000 with zero foreign policy experience a heartbeat away from the presidency," said Mr Obama's spokesman Bill Burton. But Mr Obama and his running mate, Joe Biden, congratulated Mr McCain on his choice. "It is an another sign that the old barriers are falling in our politics," they said. It is not clear that all Republicans were immediately delighted. "I have to digest this choice," said Robert Ehrlich, the former governor of Maryland. But the conservative strategist Greg Mueller dismissed early doubts. "Governor Palin is a down-the-line mainstream conservative who will energise the conservative base and reach across party lines, attracting women voters, independents and blue-collar workers."