The Bosnia flap comes at a potentially damaging time in the campaign when the New York senator is battling trust issues with the American electorate. Wednesday's ABC poll revealed that 58 percent of all Americans find Clinton not honest and not trustworthy.
On the issues of Iraq, Iran and the economy, the candidates rehashed the positions they've detailed over the last 20 debates in the Democratic presidential race, since the first primary forum took place more than a year ago.
As Gibson pointed out, the one-year anniversary of the Virginia Tech school shootings cast a shadow over the debate Wednesday, particularly as both candidates find themselves trying to balance support of gun control measures with an individual's right to bear arms.
Clinton said she respected the Second Amendment, and that it was all about finding "the right equation."
"I respect the rights of lawful gun owners to own guns, to use their guns," Clinton said, emphasizing that she "will work hard to bridge the divide which I think has been polarizing and, frankly, doesn't reflect the common sense of the American people."
Obama said it was important to figure out "what was working" among gun control measures currently in place, and that he has "never favored an all-out ban on handguns.
"I believe that the Constitution confers an individual right to bear arms. But just because you have an individual right does not mean that the state or local government can't constrain the exercise of that right," he said.
Pennsylvania's 158 pledged Democratic delegates are up for grabs in Tuesday's primary and, by national rules, are divided proportionally between the two candidates, based on their percentage win of the state.
Only registered Democrats can vote in the Keystone State's Democratic primary, and the state has seen record registration among Democratic voters and changes in party affiliation since January, because of both a tight delegate race between the Democratic contenders and an all-but-official Republican nominee.
Obama currently leads the overall count with 1,639 delegates to Clinton's 1,499. The number of delegates needed to secure the party nomination is 2,025.
If the state contests and allocation of pledged delegates do not bring the race for the party nomination to a single Democratic candidate, the Democratic race could end in the hands of the 700-some superdelegates, whose individual, independent votes count towards the 2,025 total.
Clinton said Wednesday that, if necessary, she would try to convince the superdelegates to support her with the same language she has used with American voters. "We need a fighter in the White House," Clinton said. "We need someone who's going to take on the special interests."
Obama countered that he would reiterate his campaign rhetoric for change in Washington as a means of garnering superdelegate support.
"Change does not happen from the top down," Obama said. "It happens from the bottom up ... we're going to enlist the American people in the project of changing this country."
Last month on "Good Morning America," Democratic Party Chair Howard Dean said he's in favor of letting the state nominating contests play out as necessary, but that he'd like the superdelegates to announce who they are supporting by July 1.