When the Democrat leaders in Washington were negotiating Arlen Specter’s party switch, they were eager to assure him that they would do all in their power to ensure him a clear path to the Democratic nomination for Senate. It’s becoming clear that in doing so, they promised something far outside their power to deliver.

In the 2010 race for U.S. Senate, by a 63/28 margin most Democrats believe Specter should face a challenge from one or more other Democrats in the 2010 Democratic primary in order to secure the nomination (9% are undecided). This suggests most Democrats want Specter to earn his rightful place as the Democratic nominee despite comments by Democratic Party officials and leaders that Specter should be given a free ride in the primary.

In a potential general election match-up between Democrat Arlen Specter and Republican Pat Toomey, Specter leads Toomey by a 46/37 margin (13% are undecided and 4% would support someone else). Among Republicans, Toomey leads Specter 69/21 while among Democrats Specter leads Toomey by a 65/14 margin. Among “super voter” or those who voted in 3 or 4 of the last 4 general elections the race is a near toss-up with Specter leading by a narrow 42/39 margin, while among those who voted in only 1 or 2 of the last 4 elections Specter holds a commanding 50/36 lead. This means lower turnout favors the Republican nominee in this contest, while higher turnout favors the Democrat.

Of course, all is not lost for Specter. I’m sure that of the 63 percent of Democrats who favor a primary against him, some of them must still plan to vote for Specter - perhaps as many as 10 or 15 percent. If that’s the case, he’ll only lose narrowly. (And as long as we’re on the subject, check out the latest from Mark Blumenthal, who argues that Specter is weaker than he looks based on current polls.)

Note too, the surprisingly good news for Pat Toomey. While the conventional wisdom tells us that he can’t win, he is holding Specter under 50 percent - a sign of real trouble for a well-known incumbent like Specter. Further, in a low turnout model (as 2010 is likely to be) Toomey is in a dead heat with Specter. For a challenger with relatively low name identification in the state, this poll should be extremely encouraging.

Right now Arlen Specter is probably wondering if it’s too late to switch back.