Paul Ryan flies solo for the first time as he takes on Barack Obama in a key swing state

Having fulfilled his initial task of providing a sorely needed jolt of excitement to the Republican faithful, Paul Ryan today went solo for the first time as vice presidential candidate-to-be, taking on Barack Obama directly in one of the key swing states in November’s election.

As Mitt Romney took his case to Florida, perhaps the most important swing state of all, the 42-year old Wisconsin congressman was campaigning in the midwestern battleground of Iowa – where the president was also starting a three-day bus tour, aiming to nail down a state he carried easily in 2008, but likely to be close fought this time.

The early reviews of Mr Ryan have predictably split on party lines, with Republicans exulting in a young and energetic running mate with impeccable conservative credentials. Democrats however lined up to attack him as chief architect of an austere budget-balancing plan that, they say, hurts middle- and working-class Americans, while heaping further largesse on the rich.

Mr Obama led the charge in person, describing Mr Ryan as “a decent man and a family man,” but also the “ideological leader” of the Republicans. He was an articulate spokesman for Governor Romney’s vision, “a vision that I fundamentally disagree with,” the president told cheering audience in Council Bluffs, Iowa.

Nowhere is that disagreement clearer than on Medicare, the highly popular but hugely costly federal health programme for the elderly, which Mr Ryan’s budget – passed by the Republican-controlled House but blocked in the Democrat-run Senate – would part-privatise.

Already Mr Romney has started subtly to distance himself from the proposals, insisting that while he fully backed his running mate’s goals he would be sending his own budget to Congress, if he won the White House. Today’s diverging itineraries of the pair sent a similar message.

Mr Ryan was dispatched to the Midwest, where he should appeal to white farm and blue-collar voters disillusioned with Mr Obama. But for now at least he is being kept away from Florida, thick with older voters reliant on Medicare and unlikely to appreciate his plans to overhaul the system.

The big question is how large and how lasting will be the ‘Ryan bump’ to a lacklustre Romney campaign that had been trailing Mr Obama in almost every poll – especially in key swing states like Florida, Ohio and Virginia.

The early signs were mixed: a new USA Today/Gallup poll gave him relatively modest approval ratings, probably because many voters have only a vague idea of Mr Ryan. But, with the media spotlight now fixed upon him, that will quickly change.

US election history is littered with examples of vice-presidential picks who created an initial sensation, only to fizzle – the most recent example being Sarah Palin (who today confirmed she will not even address the Republican convention in Tampa later this month).

But the Alaska governor was unknown until John McCain plucked her from obscurity in 2008. Mr Ryan by contrast has long been a leading and respected actor in Washington. In the long run however, and barring Palinesque gaffes, he probably will have little impact to the final result. On 6 November, voters will be picking a president, not a vice-president.