» There’s the Specter who’s too conservative. Rep. Joe Sestak, D-Delaware County, was poised to run against the Republican incumbent as well, challenging him from the left over his allegiance to the policies of the Bush administration.

The former admiral had planned to meet the incumbent in the November election. The forum for their confrontation has shifted to the Democratic primary, but, defying the leadership of the party they now share, Sestak’s criticism of the senator remains the same.

» There’s the Specter who isn’t as invulnerable as he once seemed. At a time when President Obama and the Democratic congressional majority enjoyed considerably higher poll ratings, Pat Toomey, a former Republican congressman, was considering a run for governor.

Then Specter, the man he had nearly ousted in the 2004 GOP primary, emerged as one of only three Republicans to support the administration’s economic stimulus plan. That vote prompted Toomey to change his mind, renew his challenge to the veteran lawmaker and, consequently, drive him out of the Republican Party.

» Finally, there’s the Specter who’s too establishment. Joe Vod Varka, a Robinson machinist, is running in the Democratic primary not so much against Specter himself but against what he sees as the senator’s membership in the broader political establishment out of touch with ordinary voters.

Specter’s decision last April to return to the party of his youth shook Pennsylvania and national politics. It prompted widespread confident — and wrong — speculation that his move symbolized the end of competitiveness for the Republican Party in the Northeast. While he frankly acknowledged that he could not win a Republican primary against Toomey, polls then showed the incumbent with a solid lead in a general election in which the independents and Democrats he had always cultivated could vote.

But amid persistent high unemployment and eroding popularity for the administration, Toomey had by the fall moved ahead of both Specter and Sestak in the polls. In two more recent surveys, however, Specter had once again recorded narrow leads.

When he crossed the Senate aisle, Specter gave Democrats a filibuster-proof majority. Brown took that away. At this point, no handicapper predicts that this year’s Senate elections will restore such a super-majority to the Democrats.

But that has not cooled the loyalty of the Democratic establishment for their new member. Obama, Vice President Joe Biden and numerous Cabinet members have paraded through the state with him over the past year. Gov. Ed Rendell has enthusiastically thrown his support behind his old friend and Philadelphia neighbor. In a recent meeting of the Democratic State Committee, Mr. Specter crushed Mr. Sestak in the party officials’ endorsement vote. And last week, state AFL-CIO officials produced a two-thirds majority to add their endorsement.

The senator isn’t resting on the value of high-profile endorsements. He has campaigned relentlessly across the state, making return visits to each of the state’s 67 counties since September.

The Specter campaign is intent on making sure that the 2010 election is not just a referendum on the incumbent. That was evident in the first ad of this campaign, aired more than a year ago, when Specter — then still anticipating a GOP primary matchup against Toomey — sought to introduce the little-known former congressman as a Wall Street manipulator mired in trading practices that produced the economic meltdown.

Polls show that most Democrats in the state haven’t heard enough about Mr. Sestak to express an opinion. But his campaign has the money to change that and to amplify his critique of the incumbent through the crucial megaphone of television ads.