On Saturday, August 11, “before the press and just about everyone else” was notified, the Mitt Romney campaign used a mobile phone app to inform supporters that Wisconsin Congressman Paul Ryan would be the Republican candidate for vice president.

Romney’s choice is significant. No vice presidential candidate has been a deciding factor in an American election since Lyndon Johnson carried Texas for John F. Kennedy in 1960. Since then the operative principle for Republicans and Democrats in selecting a VP has been the political equivalent of the Hippocratic Oath for physicians: Do No Harm. Romney was widely expected to make a “safe” choice—of former Minnesota Governor Tim Pawlenty or Rob Portman, U.S. Senator from Ohio. Instead, he has, uncharacteristically, rolled the dice.

The forty-two year old Ryan is an experienced, articulate and influential legislator, a vigorous campaigner, and the darling of conservative Republicans and Tea Party activists. A key factor in his selection is his ability to energise the party’s base, which has been less than enthusiastic about Governor Romney, a former Massachusetts moderate. Ryan’s supporters suggest as well that he will help the GOP win the 10 Electoral votes of his home state, which has been carried by the Democrats in every presidential election since the 1980s.

But Ryan’s presence on the ticket also gives President Obama an opportunity to change the conversation from the fragile, and some say faltering, economy, to a federal spending plan, authored by Ryan, that Newt Gingrich (of all people) called “right wing social engineering.” First introduced in 2010 and known variously as The Path to Prosperity and The Roadmap, the Ryan budget proposes radical changes in the role of government in promoting and protecting the welfare of American citizens – and an attack on Social Security and Medicare, two of the most popular programs in the United States.

In its current version (which passed the House of Representatives with support from all but four Republicans and died in the Democrat-controlled Senate), the Ryan budget would cut $6 trillion in discretionary spending over ten years, reducing the deficit to about 3 percent of Gross Domestic Product by fiscal year 2014, three years ahead of the White House plan. Favorable to the wealthiest Americans in virtually every provision (Ryan has signed the “no tax” pledge of lobbyist Grover Norquist, who famously aims to shrink the federal government until it’s small enough to be drowned in a bathtub) it collapses individual income tax brackets into two rates— 10% and 25% —and promises to clear out “the burdensome tangle of loopholes that distort economic activity,” without identifying any specific loophole. And it cuts corporate tax rates ten points to 25 percent while eliminating (as yet unnamed) exemptions.

Unlike a previous iteration of his plan, Ryan would not now privatise Social Security. But Democrats will surely remind voters of his earlier views — and will ask Governor Romney if he agrees with them. The latest Roadmap does phase out Medicare, a government guarantee of health care for senior citizens, in favor of vouchers, which would be used by individuals to pay for private insurance premiums – but which, according to critics, will not pay the full freight.

The Roadmap is not popular with voters. When Ryan discussed it at town hall meetings in his home state, he was shouted down. Fearing dire consequences at the ballot box, conservative columnist Charles Krauthammer characterized the Ryan Plan, which weighed in at 73 pages with 37 footnotes, as perhaps “the most annotated suicide note in history.” Until recently, Romney, who has endorsed The Roadmap, kept his distance from it.

He will not now be able to do so. And so, the selection of Ryan will make it far more difficult for Republicans to carry Florida, a must-win state with many senior citizens – and 29 Electoral votes.

As Democrats assert that, by jumping on Ryan’s Express, Romney has demonstrated that he is a hostage to Tea Party extremists, they may also take note of Ryan’s record during the presidency of George W. Bush, when he voted for sweeping tax cuts, a costly prescription-drug entitlement for Medicare, two wars, and the multibillion-dollar bank-bailout legislation known as TARP, and in so doing, helped add $5 trillion to the national debt. Ryan now claims that the experience made him miserable. The GOP faithful will give him the benefit of the doubt, but mainstream voters may be less forgiving.

Whether Ryan’s presence on the ticket proves a boost or a bust, it could inject a substantive discussion of the issues into what hitherto has been a dispiritingly vicious contest. Congressman Ryan could save both Romney’s campaign and his own efforts to privatize the economy. He could also sink them both.