Serious, sober and grounded. Sen. Rob Portman (R-Ohio) is all those things. But he is not on the GOP ticket. At the convention, he sat with a group of journalists from The Post and  Bloomberg to answer dutifully questions on likability (Mitt Romney’s), the debt (“We’re in real trouble”), the lack of specificity on “base broadeners” in Romney’s tax plan and Medicare proposal. It was a reminder of how dramatically different the presidential race would have been had he and not Rep. Paul Ryan (R-Wis.) been in it.

Portman is optimistic that Romney can win Ohio. But certainly his presence on the ticket would have been worth a point or two. Wisconsin might be less in play than it is with its native son on the bottom of the ticket. Portman likely would not have launched the Romney campaign with a preemptive attack on Medicare. Nor would there be endless questions about the Ryan budget. But by the same token, it is not clear how much Portman would have aided Romney in sharpening the debate and moving the race swiftly back to fiscal issues.

Portman is arguably what Romney-Ryan will need in the Senate if the ticket wins. On the Armed Services and Budget committees, the former head of the Office of Management and Budget would play a critical role in transforming campaign promises into legislation. He was hammered by the journalists about Romney’s failure to specify the exact base broadeners in his tax plan. For now, Portman only hints at a pick-and-choose formulation (“You can keep charitable deductions but not the mortgage deduction on the second home.”). In the Senate, it is he and other Republicans who will be charged with figuring out, in heated negotiations with other lawmakers, what specific deductions will be sacrificed. He is not as rhetorically sharp, nor wedded to budget specifics, as Ryan is, but he exudes grown-upness. He seems like the guy to figure those compromises out.

On sequestration, he conceded that his vote in favor of the debt-ceiling deal was likely a mistake. The “trigger,” he said, “was not the right structure.” With the easy way out to let across-the-board cuts go forward, the supercommittee faltered. “It was better to have a cliff,” he said. As a member of the supercommittee, he backed a plan that included both revenue-generating tax reform and entitlement reform. If the Republicans win the Senate and the White House, you can imagine that formulation would be revived.

Ryan has rallied the base, taken the fight to the president on Medicare, put new states in play and turned out to be an exciting stump speaker. In that regard, the pol who made his name as a budget wonk has, ironically, turned out to be a great political choice for Romney. But he and Romney, if elected, will need to convert ambition into policy. In that regard, perhaps it is better to have Portman in the Senate and Ryan in the White House rather than Portman in the White House and Ryan in the House.

Portman also highlighted the message difficulty the GOP ticket has. He was peppered with questions that assume Romney is the delinquent on policy (What about President Obama, who has no comprehensive tax plan, no entitlement plans?) and  that Ryan’s acceptance of the Obama Medicare cuts will rule the day. (Ryan has repeatedly said he accepts Romney’s plan to put back in the $700 million taken out for Obamacare.) The media is certainly convinced likability is Romney’s big problem (although national and most state polling has the race tied). Portman tried to gently steer the conversation back to Romney’s message. But in the hot, hostile 24/7 media environment, perhaps the more rhetorically aggressive Ryan is precisely the right figure to drive the message.