Paul Ryan's big moment arrives, but can he deliver?

Paul Ryan will deliver a speech to an audience of between 20 and 30 million Americans (Getty Images)

Paul Ryan – once a little known Wisconsin congressman who specialised in the federal budget – will complete a remarkable political ascent tonight when he accepts the nomination to be Mitt Romney's running mate in the Republican bid to retake the White House in November.

At a send-off rally in the gym of his old high school on the eve of his arrival in Tampa, Mr Ryan offered a taste of what he will be telling convention delegates today, urging cuts in both government spending and taxes to revive the country's flagging economy.

Calling President Obama's America "a nation in debt, a nation in doubt, a nation in decline," he pledged that a Romney administration would not dodge the tough issues, as Republicans accuse Mr Obama of doing. "We are going to lead" he declared.

The choice of the 42 year-old Mr Ryan has sent a jolt of excitement through the party's conservative wing, still unconvinced that the party's presidential candidate is truly one of its number. That boost in morale has not yet been reflected in the polls – most of which show Mr Obama still narrowly ahead – but it has increased Republican chances of carrying Wisconsin, a state that Republicans last won in 1984.

Tonight however, Mr Ryan must deliver the red meat conservatives expect without putting off moderates among the 20 to 30 million Americans likely to watch his acceptance speech on television.

Two problems dog him. One is abortion, where their co-sponsorship of bills that would give full legal rights to a foetus from the moment of conception has tied Mr Ryan to the Missouri Senate candidate Todd Akin, whose remarks about rape earlier this month caused uproar.

The other is his proposal to part-privatise Medicare, the popular but extremely costly government healthcare programme for the elderly.