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Out of America: Romney's choice of Paul Ryan as his running mate in the US elections is exciting, but its consequences are unpredictable

Presidential? Mitt Romney, right, introduces his running mate, Paul Ryan, yesterday (EPA)

For once, Mitt Romney has been bold. With yesterday's announcement of the 42-year-old Wisconsin Congressman Paul Ryan as his running mate, the Republican presidential candidate has confounded many expectations. The setting for the great unveiling may have been predictably patriotic: the deck of the mighty Second World War battleship USS Wisconsin, now a museum at the naval base of Norfolk, Virginia. But his choice is genuinely exciting and its consequences very much unpredictable.

Who is Paul Ryan? And, more important, can he make a difference? Historically, running mates have rarely had much impact on a campaign. Americans vote for presidents, not vice presidents. Richard Nixon won twice despite having the mediocre Spiro Agnew in the ticket; ditto George HW Bush and Dan Quayle. The most important rule is, do no harm – a precept violated by John McCain when his selection of the shockingly unqualified Sarah Palin undoubtedly contributed to his defeat four years ago.

From that perspective, Ryan, untested and with little experience in foreign affairs, is a calculated risk. The upside is obvious. He is young, personable and articulate, with the obligatory attractive family. Long seen as a Republican superstar in the making, he embodies the party's future rather than its past (as Romney's two most touted "safe" options of either Tim Pawlenty, former governor of Minnesota, or Ohio's Senator Rob Portman, might have been portrayed). He is his party's acknowledged expert on the budget and matters economic, respected across the aisle in a notoriously polarised Congress. Just conceivably, too, he could make a difference in his home state of Wisconsin, usually Democratic but which Republicans have an outside chance of carrying in November.

More broadly, he makes perfect ideological sense. Ryan's deficit-cutting zeal endears him to a resurgent Tea Party movement. His credentials as a social conservative, on issues from guns to abortion, are impeccable and will thus bolster Romney among many activist Republicans still wary of him. A Romney/Ryan ticket ensures the party will march united into the critical final three months of the campaign.

Above all, after weeks overshadowed by controversy over the candidate's personal tax arrangements and by his gaffes on his trip to Britain, Israel and Poland, the argument is squarely back to where Romney wants it: the economy. True, yesterday brought another momentary stumble, when the candidate introduced his running mate as "the next President of the United States". But Romney smilingly and speedily corrected himself, and in his first speech in his new incarnation, Ryan laid out the ground on which the campaign will be fought.

Sure, President Obama had been dealt a tough hand, he acknowledged. But despite having total control of Washington for his first two years in the White House, he had delivered "the worst economic recovery in 70 years". Americans were plagued with "debt, doubt and despair". Whatever else, Ryan's selection guarantees that when they vote on 6 November, Americans will have a stark choice between two very different visions of the future.

And thereby hangs the biggest risk. In the days before Romney made his announcement, influential Republican opinion-makers such as The Wall Street Journal's editorial pages and The Weekly Standard magazine (also owned by Rupert Murdoch) were singing Ryan's praises. But the very reasons that have the Journal drooling – the Congressman's sweeping proposals to cut the deficit and his embrace of unfettered markets – also have Democrats licking their lips.

Ryan's draft budget, which was voted through by the Republican-controlled House of Representatives in March, would cut top tax rates, both corporate and personal, to 25 per cent from today's 35 per cent, in stark contrast to President Obama's call to repeal the Bush tax cuts for high earners. Most controversially, he would take an axe to the federal public health-care programmes Medicare and Medicaid, part-privatising the former, and handing control of the latter back to individual states.

No one seriously disagrees that both have to be scaled back, if a deficit currently running at close to 10 per cent of GDP is to be brought under control. However, Medicare and Medicaid are highly popular. Not only did Republicans in Congress embrace the Ryan plan – so did Mitt Romney himself, calling it "marvellous". However likeable, Ryan is a polarising figure, and Democrats will portray him as proof that Republicans are the callous party of the rich, indifferent to the less well-off, still fixated on the trickle-down economics that, in Obama's words in a Democratic TV ad running now, "got us into this mess in the first place".

From Romney's perspective, however, the potential rewards of his selection outweigh the risks. This has been a bad summer so far for the Republican candidate. Despite the struggling economy, he remains – narrowly but consistently – behind in the polls, trailing Obama in almost all of the swing states that will decide the election, including Wisconsin. His campaign has been listless, allowing Democrats to define their opponent as the out-of-touch corporate vulture who led Bain Capital, laying scarcely a glove on a vulnerable President.

Is Ryan the proverbial "game-changer"? Perhaps not, given the rarity with which vice-presidential nominees actually change the game. But 2012 could be an exception. So polarised is the electorate, polls suggest, that only 5 per cent or so of Americans have yet to make up their minds. The key, the pundits now say, lies less in chasing a vanishing pool of genuine undecideds than in firing up the party base, and making sure the already converted go to the polls on election day. For that task, Ryan could be ideal.

Mitt Romney is instinctively risk-adverse. He likes to leave nothing to chance, and for his investments to pay off, handsomely. In the past few weeks, he has pored over the vice-presidential data, and run it though the political number crunchers. On 1 August, immediately after his return from his contentious foreign trip, he is said to have made his decision. Yesterday, he put his money on the table. For the time being at least, Romney has recaptured the initiative. And, just possibly, the Ryan investment will earn him the biggest prize of all.