As has emerged as the frontrunner for the Democratic presidential nomination, voters have been offered a preview of how Republicans will likely characterize the Illinois senator in a general election match up.
One recurring theme has been where Obama falls on the ideological spectrum.
In early February, Republican National Committee spokesman Alex Conant called Obama the Senate's "most liberal" member. Presumptive GOP nominee, Newt Gingrich and others have echoed the charge.
Their evidence? A January 31st finding by National Journal that put Obama at the top of the list of most liberal senators in 2007.
Obama's critics don't always mention the fine print: Obama was found to be the 10th most liberal senator the previous year and the 16th most liberal the year before that, and other studies don't echo National Journal's findings.
Nonetheless, the "most liberal" charge is likely to be heard early and often should Obama emerge as the Democratic nominee, particularly as it appears at odds with his insistence that he will reach across the aisle and shift the political calculus away from the bitter antagonism of recent years.
"There's a lot of uncertainty about how he would govern and how he would put into practice what we think of as bipartisanship given what we know about his Senate record," said Sarah Binder, a senior fellow at the Brookings Institution and an expert on Congress.
Binder notes that Obama has spent his time in the Senate on "midrange issues," such as ethics reform and nuclear proliferation, where he could more easily find a Republican senator that would be open to working with him.
"He wasn't working on issues where you have hard Democratic vs. Republican divides," she said. "He seems to have very carefully chosen out areas where there were prospects of success and where he could find like-minded people across the aisle."
"He's been cautious, overall, as a senator," said David Epstein, a professor of political science at Columbia University. "He hasn't been a firebrand in terms of liberal causes, but he's staked out a leftist policy position. His record doesn't provide a lot of basis for the claim that once he's president he's going to win over Republicans and sort of bring everyone together."
Democratic Illinois State Senator Denny Jacobs, who played poker with Obama when they were both state legislators, disagrees.
"He brought some ethics reform to Illinois, and that's no easy thing," said Jacobs. "He worked with both sides of the aisle to get that done. I think he's pragmatic. Illinois is the wild west of campaign finance, and Barack came in here and created some rules."
"I think that what he's proven is that he does believe in something, he does have a core, but he's also someone who can move people from both sides to a resolution," Jacobs added.
Republican Illinois State Senator Bill Brady, who also worked with Obama, said Obama was "probably the most liberal member of the Illinois State Senate" - in part because he "represented the most liberal area in the state." (Epstein, using roll call vote and bill co-sponsorship data, found Obama to be the 6th most liberal in the State Senate.)
"I think he's a believer," said Brady. "He's a bleeding heart liberal, but he's smart enough not to put it in people's face right now."
In a debate withlast Thursday, Obama was asked to respond after his rival brought up an uncomfortable cable-television exchange in which an Obama supporter was unable to identify any of the senator's accomplishments. Obama cited his work on health care, providing tax breaks to families, reforming the criminal justice system, and passing "the toughest ethics reform legislation since Watergate."
"I don't perceive him as one of the chamber's liberal members," said Jennifer Duffy, a veteran analyst with the Cook Political Report. "I don't perceive him as one of the guys who goes down on the floor and rails about his pet cause."
Obama has been criticized for not taking a stand early in his career on some hot-button issues, including abortion: He voted "present" on abortion questions seven out of 14 times in the state Senate, including once when the issue was a statewide ban on so-called partial-birth abortion.
Binder said Obama's time in the Senate suggests that he seems "comfortable working across the aisle on some of these smaller type of issues." But she suggested there are few clues as to how effective President Obama would be in dealing with larger, more divisive issues such as the war, health care, and entitlement reform.
"Those are all issues where there could be bipartisan solutions, but it's a big challenge," she said.