He and former Minnesota Gov. Tim Pawlenty are at the top of Mitt Romney's vice presidential list, but Ohio Sen. Rob Portman's ties to former President Bush's trade and budget policies are raising red flags in Boston that picking him would open the doors to attacks from Team Obama that Romney is just a Bush clone.

"The American people made the decision on Bush and they rejected him in 2008," said a campaign associate. "That would just remind voters of Bush and what they didn't like about him."

In 2005, Portman became Bush's trade czar at a time when the administration was bragging about how jobs outsourcing would have a positive economic benefit. He later became Bush's budget director at a time of growing deficits.

The Obama campaign has not been directly tying Romney to Bush, but has eagerly looked for ways to suggest that the Republican wants to return to Bush-era policies. They have been pointing out that Romney's tax plans were tried under Bush and didn't create the promised jobs. But, they say, picking Portman would change that narrative.

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That worries some Romney associates. "They can make it sound like we're Bush all over again and it puts us on defense," said another campaign associate. "Same thing with Condi Rice. We'd have to debate the whole weapons of mass destruction and war again. We're on offense on our own issues now and have to stick with that."

They note that Bush has decided against attending the Republican National Convention in part because he doesn't want to revive any remaining animosity toward him and his policies.

A Portman associate, however, said that he remains at the top of the VP list because he does have a good record from the Bush years to brag on, and is a popular senator from a state Romney needs to win.

Portman, who beat back the charges in winning the Senate in 2010, has taken on the issues of his time with Bush, stating that the one budget he prepared was balanced.

Also, his team said that his time as U.S. Trade Representative was highlighted by his challenging China. Said a source close to Portman, "Democrats ran a campaign in 2010 on these issues, and they lost by 18.5% in the most purple state in the union, with their message failing to resonate with anyone more than national reporters parachuting in to Ohio in the middle of the campaign. Rob was the first USTR to successfully take China to court and win, and Ohio auto parts workers were more competitive because of it. He was the first trade representative to highlight Chinese currency manipulation as a trade issues, which is why he continues to urge the Obama administration to step up and call the Chinese currency manipulators, which they've failed to do on seven different occasions."

And on Thursday, Larry Sabato, who heads the University of Virginia's Center for Politics, named Portman and Pawlenty at the top of his final vice presidential prediction list, though he cited Portman's Bush ties with this note: "held responsible for budget problems."

But the heartburn over Portman has raised hopes for Pawlenty, Florida Sen. Marco Rubio and Louisiana Gov. Bobby Jindal. "They don't have any Bush ties and they also have a good story to tell," said a campaign advisor.