THE BEST THING about Mitt Romney’s choice of Wisconsin Republican Paul Ryan to be his running mate is that it offers the chance to transform what has been a dismally substance-free campaign into a serious clash of ideas. The energetic, likable House Budget Committee chairman has made himself into the party’s leading thinker on the fiscal and budgetary matters that will confront the next president. We have differed sharply with Mr. Ryan’s policy proposals, which would cut far too deeply into an already fraying social safety net and raise too little revenue to support the needs of an aging society. But Mr. Ryan has demonstrated a willingness to tackle third-rail issues from Social Security (he has been a leading proponent of instituting voluntary private accounts, raising the retirement age and reducing benefits for better-off seniors) to Medicare (he would transform it, most recently in a plan worked out with Oregon Democratic Sen. Ron Wyden, into a voucher-like system under which seniors would purchase coverage on their own).

That has made Mr. Ryan an influential player in the Washington debate and makes him a welcome addition to the 2012 race. The 42-year-old Mr. Ryan has been something of a one-man idea factory: On taxes, he has proposed letting taxpayers choose between the existing system and a new one that would have just two rates, 10 percent and 25 percent, and would eliminate taxes entirely on investment income; in a move that seems guaranteed to drain money from the Treasury, taxpayers would be able to choose between that and the existing system. On health care, he has endorsed doing away with the tax-free treatment of employer-sponsored health insurance and replacing it with a refundable credit. There are major flaws and omissions in Mr. Ryan’s cornucopia of proposals. Yet his selection puts useful pressure on both Mr. Romney and President Obama to be more specific about their own approaches to entitlement spending, tax reform and other budgetary issues about which they would prefer to speak, if at all, in vague generalities.

Bold is not normally a word associated with Mr. Romney, but this is a bold — perhaps even foolhardy — choice. The clash of ideas will inevitably be distilled into a 30-second caricature of Mr. Ryan’s plans, no doubt targeted to Florida seniors who would be exempt from the proposed Medicare changes. Indeed, Mr. Obama has already used his opponent’s support of the Ryan budget as a campaign cudgel. “He even called it marvelous, which is a word you don’t often hear in describing a budget,” the president said in an April speech, proceeding to dismiss the Ryan plan as “thinly veiled social Darwinism,” “antithetical to our entire history as a land of opportunity and upward mobility for everybody who’s willing to work for it” and “a prescription for decline.” Mr. Ryan’s contrasting vision, laid out most usefully in his January 2010 “Roadmap for America’s Future,” decries an “expanding culture of dependency” on government programs. “By this means,” Mr. Ryan wrote, “government increasingly dictates how Americans live their lives. The process suffocates individual initiative and transforms self-reliance into a vice and government dependency into a virtue.” This dark assessment mischaracterizes the much-diminished nature of the modern safety net, but it illustrates the passion underlying Mr. Ryan’s policy views.

Mr. Romney’s selection of a running mate sharpens the portrait of the kind of president he wants to be. It underscores the stark choice facing voters in November.