Editor's note: William Howell is the Sydney Stein professor in American politics at the University of Chicago.
(CNN) -- One and only one argument has the potential to deliver Mitt Romney and Paul Ryan to the White House. The argument comes in two parts, the first of which is perfectly familiar.
The only way to address our mounting debt and put back to work the millions of Americans either jobless or underemployed is to grow the economy. Businesses large and small must recover a level of security and freedom to expand their operations, make capital investments and grow their workforce.
It's the second part, though, that Romney needs to drive home tonight: Why he can be trusted to make that growth happen.
Additional public spending does not present a sustainable policy platform. The nation's deficits are simply too great and the returns on public investments too low for the federal government to be able to spend its way out of this economic malaise. Alone, those austerity measures that stand any chance of enactment -- and here, Paul Ryan, take heed -- will register only passing blows to the federal government's extraordinary financial obligations. If Europe offers any lesson, it's that serious drawdowns on the federal budget threaten whatever economic recovery is under way.
Growth, and growth alone, promises a way forward. Unrelated discussions about abortion, immigration, White House leaks, the dysfunctions of Washington or patriotic sentiment merely distract from the only argument that matters. Additionally, some of these discussions genuinely threaten to fragment the Republican Party and alienate swing voters. Meanwhile, every pertinent discussion about financial regulation, the tax code, international trade and entitlement overhaul must reinforce a central message of growth.
So far, so good. But the argument cannot end there if Republicans hope to reclaim the presidency. As Ryan himself admitted Wednesday night, "By themselves, the failures of one administration are not a mandate for a new administration. A challenger must stand on his own merits."
And so the second part of the argument, and the one that needs shoring up the most during this convention, is that Romney and Ryan offer the best chance for economic recovery. The Republican Party must give the American public reasons to believe that their ticket offers the leadership needed to grow the economy.
In the first two nights of this convention, they have not.
Repeatedly intoning that "we can do better," as a steady stream of convention speakers have done, will not suffice. The question is not whether "we" can do better, but whether Romney can do so, whether there is something in Romney's personal history, character or vision that can bring about the kind of economic recovery that, so far, has eluded the current administration.
Although delegates dutifully applauded to scripted lines, the speakers offered little of lasting value, little that undecided voters could hold on to when the Democrats have their turn next week.
Ryan, to be sure, had his moments. He proved most effective when he lambasted President Obama not just for having enacted a health care reform that, in his view, amounts to "2,000 pages of rules, mandates, taxes, fees and fines that have no place in a free country," but for insisting that we, as a nation, engage a long, divisive debate on health care amid a financial crisis and massive unemployment.
Ryan also did a nice job of conveying his allegiance with the owners of "restaurants, cleaners, gyms, hair salons, hardware stores," and of signaling his appreciation for the hard work and dedication required to run a small business.
And in perhaps his best line of the evening, Ryan recognized the disaffection of the youth who have been most hurt by the economic downturn. "College graduates should not have to live out their 20s in their childhood bedrooms, staring up at fading Obama posters and wondering when they can move out and get going with life."
But much of Ryan's speech, following on the budgetary reforms that launched him to national prominence, centered on the debt crisis in isolation. Ryan's critique of Obama's stimulus package started and ended with the observation that "money wasn't just spent and wasted -- it was borrowed, spent and wasted." But Ryan did not do nearly enough to spell out why this matters for economic growth, or what Romney would have done differently.
Ryan's subsequent reflections on faith and family, his insistence that individual rights are afforded by God and nature rather than government, and his thoughts on the nation's "moral creed" may have stirred the Republican base, but it did little to sharpen the singular message that stands some chance of tipping the election.
He made passing reference to Romney's "rescue" of the 2002 Winter Olympics, but he did not produce any examples of companies that Romney resuscitated while working in the private sector. He did not offer specifics on which, if any, of Romney's dealings at Bain Capital improved the lives of workers and the long-term prospects of their employers.
While recognizing that Romney as governor of Massachusetts balanced the state's budget without raising taxes, Ryan also did not describe how that bluest of New England states grew economically under Romney's watch.
Ultimately, of course, others cannot make the argument on his behalf. Romney must deliver it himself. All of the speeches we have heard so far are mere warmup acts to Romney's time behind the podium.
The stakes couldn't be higher. If Romney does not succinctly and powerfully state that growth is essential, and then personally and movingly address how he would do that, Obama may well become the first president to win re-election since Franklin Roosevelt when the unemployment rate stood above 8%.
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The opinions expressed in this commentary are solely those of William Howell.