In Tuesday night's Democratic primary debate in Philadelphia, Sen. Hillary Clinton was challenged by her competitors for the nomination, notably former Sen. John Edwards and Sen. Barack Obama.

More aggressive questioning is not surprising, given Clinton's substantial lead in poll numbers and campaign dollars. Edwards and Obama have to try to close the gap if they are to have a chance.

But this week's debate appeared to be more than just desperate contenders taking shots at the leader. Edwards and Obama raised issues that have dogged Clinton in recent months, such as her tendency to take both sides of an issue, her ties to big-money interests in insurance and finance and also the secrecy surrounding her papers at the Clinton Presidential Library.

Clinton's reluctance to take a firm stand Tuesday night on how she would reform Social Security or whether or not she supports New York Gov. Eliot Spitzer's plan to issue driver's licenses to illegal aliens can be partly attributed to the natural tendency of a leading candidate to want to avoid mistakes or get pinned down. But in Clinton's case, many see a continued pattern of finessing answers that was also a potent skill of her husband, former President Bill Clinton. Telling everyone what they want to hear and coming down on both sides of an issue works, but only to a point.

After initially expressing support for Spitzer's plan Tuesday night, she then said she would not support it.

Obama quickly spoke up, saying, "I can't tell whether she was for it or against it." He then pointed to Clinton's shifts in positions on the war in Iraq, the North American Free Trade Agreement and other issues.

Edwards, who was seen as Clinton's stongest challenger Tuesday, suggested her for-it-and-against-it spin on the Spitzer plan was just the latest example of her "double talk."

Listening to her position on her governor's plan for licenses for illegal aliens, Edwards said, "Unless I missed something, Sen. Clinton said two different things in the course of about two minutes."

Given her criticism of the Bush administration for its secrecy and lack of transparency, Clinton took a few hits in this area too Tuesday night. As a candidate, she boasts of her experience in the White House, aiding her husband during his eight years as president. Yet, the Clinton presidential library is refusing to release documents related to her activities in the White House. Officials at the library, citing a directive from the former president, have not acted on 287 requests filed under the Freedom of Information Act related to Sen. Clinton's time as First Lady.

President Clinton has ordered that none of those documents be released until 2012.

Debate moderator Tim Russert asked Sen. Clinton if she would push to have those documents released prior to the election. She declined to answer, saying only "we'll move as quickly as our circumstances and the processes of the National Archives permits."

Obama picked up on the issue saying, "As you're making the claim that this is the basis of your experience, I think, (this) is a problem."

No contenders landed any knock-out blows on Clinton Tuesday. But they have begun to ask critical questions regarding her forthrightness, transparency and ties to the status quo in Washington, D.C.

As the front-runner, Clinton has been skillfully protecting her lead. But Tuesday night's debate suggests she is vulnerable in several areas, areas that are sure to be probed in the coming weeks.

The headline of the story on the debate said, "Dems gang up on Hillary." And though Tuesday night's informative give-and-take can be labeled as ganging up on the leader, it also was about trying to get a candidate for president to answer questions and reveal more about her philosophy and ethics.

Whatever it's labeled, that's a healthy part of the process. Voters should look at instances of her so-called double talk — and they should expect her to take firm stands on issues.

They also should expect answers as to why her correspondences while in the White House are being kept under lock and key. And for those voters wanting real change, the issues of Clinton's ties to the traditional power structure and campaign finance contributors might also be an issue.

Edwards and Obama are clearly becoming more aggresive in their efforts to derail the Clinton juggernaut.

With primary season rapidly approaching, the Democratic contest finally is getting interesting.

And that's the way it should be. The leading candidate must be scrutinized so that voters know what they will be getting.