Cautious Mitt Romney rolled the dice Saturday with the selection of Rep. Paul Ryan of Wisconsin as his vice-presidential running mate. Ryan will energize a conservative base that has been slow to warm to Romney, but Democrats were elated by the choice as well. There was no one on Romney’s short list of contenders they wanted to run against more than the chairman of the House Budget Committee.

The selection of Ryan, the architect of a sweeping and controversial budget blueprint, signals that Romney may now believe that relying on the economy’s weakness alone will not be enough to defeat President Obama, particularly with new polls showing the president leading after months in which he and Romney were in a statistical dead heat.

Ryan’s addition to the ticket shows that Romney is prepared to run a more robust campaign with a sharper message built around tax and spending cuts, deficit reduction and entitlement reform. That is exactly what a growing chorus of Republicans, nervous about the direction of the Romney campaign, has been urging.

A Romney-Ryan ticket will help to clarify the choices for voters in November. Rarely have the two parties presented such a stark contrast in visions as now appears to be the case. Those competing visions could produce, after a summer of often small-minded tactics, the kind of big debate about the country’s future that both Obama and Romney have said this campaign should be about.

Such a debate will generate as much heat as light, however, which is the risk that comes with putting Ryan on the ticket. Romney has now assumed ownership of Ryan’s budgetary plan and its provisions for reining in the cost of entitlement programs. Democrats will attack it and its author as vigorously as they have tried to savage Romney’s business background and personal finances.

Ryan’s proposed changes to Medicare—he would partially privatize the program—will become the principal focus of those Democratic attacks. A new survey conducted by the Washington Post and the Kaiser Family Foundation found that 58 percent of Americans want Medicare left alone. The poll, which examines in detail the internal divisions in both parties, found that only one of five Republican groups—those who most strongly support the tea party movement—favor something like the changes Ryan has advocated.

That underscores the risk that goes along with selecting Ryan. But the congressman from Janesville, Wisc., brings clear attributes to the campaign ahead. Romney may be the presumptive Republican nominee but Ryan is the intellectual leader of congressional Republicans and to a great extent the party as a whole. He is from the party’s young and rising generation, which has a surer sense of the party’s new identity than Romney.

One sign of the degree to which conservatives look to Ryan came in the days leading to the announcement. Ryan’s candidacy was promoted by major elements of the conservative opinion makers, including the Wall Street Journal editorial page, the Weekly Standard and the editor of National Review. They will now get behind Romney’s candidacy with more enthusiasm than they’ve shown in the past.

Ryan will make the case for economic prescriptions that include sharp cuts in spending along with tax cuts and entitlement reform more passionately than anyone else, and as a former staffer to the late Buffalo congressman and 1996 vice presidential nominee Jack Kemp, with a projection of optimism not austerity.

Ryan has other attributes that commend his choice. He is a conservative Roman Catholic in a year when the Catholic vote could be pivotal in a number of battlegrounds, particularly in his native Midwest. He comes from Wisconsin, a state Democrats have won in every election dating to 1988. If the Romney-Ryan ticket were to prevail in the Badger State, that would help scramble some of the Electoral College calculations at Obama’s Chicago headquarters.

Ryan has the potential to make Romney a better candidate. Anyone who has watched the two men campaign together has seen the chemistry that exists between them. With Ryan on the stage next to him, Romney is more animated and relaxed and seemingly comfortable in having Ryan add firepower and heft to his message about the economy and the deficit.

Up to now, Romney has run a campaign that has been criticized for being too cautious and constrained. His strategy has been grounded in his and his advisers’ belief that the economic record of the past four years leaves Obama more than vulnerable. But many Republicans have challenged the Romney campaign to offer a bolder platform for the economy.

Among them was Wisconsin Gov. Scott Walker, who survived a tough recall election in June by forcefully defending controversial budget and economic policies that triggered months of protests and a succession of recall elections. After winning the recall, Walker said his victory was proof that voters will reward leaders to make bold decisions and stick to their convictions.

Ryan is among those who have urged Romney to adopt a similar campaign strategy. “We can’t just win by default, by beating up on Obama,” he told me a few months ago when I asked him what it would take for Republicans to win the White House. Of Romney, he said, “He’s got to go to the country with what I call the choice of two futures. Not just vague platitudes but [to say] this is the path the president’s taking us down and this is where I want to take us and here’s how I want to get there.”

Ryan already has tangled with Obama and the White House. After Ryan issued his first budget in 2011, Obama savaged it, with Ryan in the audience. The president said the adoption of such a plan would lead to an America that would be fundamentally different than what we’ve known throughout our history. Last spring, he described the Ryan plan as “thinly veiled social Darwinism” and a “radical vision” that is “antithetical to our entire history as a land of opportunity and upward mobility.”

Romney had other good choices among those on his short list. Both Ohio Gov. Rob Portman and former Minnesota governor Tim Pawlenty, two seasoned politicians, would have added something helpful to the GOP ticket, though neither seemed to excite the party. Each believed through much of the process that Romney would pick the other.

That Romney turned to Ryan in the end says much about his competitiveness and his willingness to take risks. Risk-taking was always part of Romney’s approach to business, but that has not been the case so much in this campaign. Picking Ryan will be interpreted inside the GOP and beyond as evidence that Romney believed he needed to shake up the campaign as he looks to his convention, the debates and what will be an intense fall campaign.

The Republicans have been hungering for a presidential campaign that would draw bright lines with the president. They want a nominee who will challenge Obama to defend what they see as a lack of leadership not just on the economy but also on the fiscal problems that they believe threaten the country’s future. With Ryan at his side, Romney has now decided to run that kind of campaign, with all the benefits and costs that come with it.