“YOU ARE ENTITLED to the clearest possible choice because the time for choosing is drawing near,” vice presidential nominee Rep. Paul Ryan told the Republican National Convention in Tampa in his hard-hitting acceptance speech Wednesday night. “So here is our pledge: We will not duck the tough issues — we will lead.”

Those are fine words; we have heard the sentiment before, including from the incumbent president. But if Mr. Ryan and Mitt Romney want credit for not ducking, and if they truly believe that voters are entitled to the clearest possible choice, it would behoove the candidates to offer more details about what, precisely, voters are choosing.

That, however, wasn’t on Mr. Ryan’s agenda. Instead he offered a speech that was part introduction of himself and his small-town origins, part testimonial to his running mate and — in largest part — a slashing and, in many elements, misleading indictment of President Obama as both a spent force and a threat to American freedom. Mr. Romney and Mr. Obama have starkly different visions about the role of government, but to caricature the president’s vision as “a government-planned life, where everything is free but us” insults voters who surely know better. Emblematic of the liberties Mr. Ryan took was his depiction of the hometown auto plant whose shuttering he implicitly blamed on Mr. Obama — even though the plant closed before the president was inaugurated.

A convention speech is not a budget submission, even when, as with Mr. Ryan, it comes from the chairman of the House Budget Committee. But a party that claims to be willing to make hard choices ought to be prepared to spell some of them out. Mr. Ryan offered only the bare assertion that federal spending of 20 percent of the gross domestic product is “enough” — despite the aging of the population and Mr. Romney’s vow to keep defense spending alone at 4 percent.

Mr. Ryan has been an intellectual leader of his party on fiscal issues, and Mr. Romney’s decision to add him to the Republican ticket represented an opportunity to focus the debate on this urgent matter, “before,” as Mr. Ryan said Wednesday, “the math and the momentum overwhelm us all.” Mr. Ryan skewered the president in his speech for creating and then walking away from a bipartisan debt commission that, he said, “came back with an urgent report.” We’ve expressed similar frustrations, but omitted from Mr. Ryan’s self-serving rendition was the uncomfortable fact that Mr. Ryan served on that very commission but was unwilling to follow the brave lead of the Republican senators on the panel who supported its “urgent” recommendations. Will the Romney-Ryan ticket endorse them now?

Mr. Ryan’s selection prompted a serious discussion of Medicare reform but also ushered in a depressingly predictable series of “Mediscare” charges and counter-charges. Mr. Ryan stooped to some of that Wednesday night, asserting that “the greatest threat to Medicare is Obamacare,” although the health care law began the hard task of reforming the program. He assailed Mr. Obama for having “funneled” $716 billion out of Medicare, without mention that his own budget assumed cuts of precisely that magnitude.

Mr. Ryan described Mr. Romney as a man prepared “to meet serious challenges in a serious way, without excuses and idle words.” Mr. Romney’s appearance before the convention Thursday night is an opportunity to demonstrate that seriousness — matching rhetoric with substance appropriate to the magnitude of the task the next president will face.