The two-term senator and two-time presidential candidate from Colorado said he recently discussed a possible bid with national and state party leaders who urged him to jump in.
Hart previously refused to comment directly on the race.
"I was called by the national party leadership and the state party leadership saying, would I consider it," Hart said in a telephone interview with The Associated Press. Hart said he would reach a decision "sooner rather than later, but I don't know how much sooner is."
Hart said he deferred last fall to U.S. Rep. Mark Udall (search), D-Colo., who ultimately decided against running.
Hart, 67, has discussed the race with New Jersey Sen. Jon Corzine (search), chairman of the Democrats' Senate campaign organization, Senate Minority Leader Tom Daschle, and other national and state party leaders.
Democratic leaders believe Campbell is vulnerable despite winning 62 percent of the vote in 1998. The former Democrat recently was treated for prostate cancer, but in announcing his candidacy insisted he is healthy and ready for a vigorous campaign.