He, like Marco Rubio, is from key swing state, and he has a strong budget and finance background – he was head of the Office of Management and Budget in the George W. Bush administration. He’s well liked in the Republican establishment.
Of course, as part of the Bush administration he takes criticism from some Republicans for helping the national debt to balloon. Senator Portman also may be a little too safe. Instead of helping to round out Romney’s weaknesses, he’s very similar to Romney – but that may also increase Romney’s comfort with him as a running rate.