I like the National Jewish Democratic Council's take on this news story better than anyone's:

NJDC welcomes Pennsylvania Senator Arlen Specter to the Democratic Party.

The Democratic Party of 2009 is an inclusive party that believes political parties should be big tents. In contrast, the Republican Party of 2009 is a party which believes in circling the wagons into an ever smaller and smaller circle until only the ideologically pure are welcome. Senator Specter has been a Republican since 1966 but he has never been a conservative ideologue. It has become increasingly clear in recent years that he is not ideologically pure enough for the Republican activists of today.

The Republican Party’s continued shift to the right has had negative implications for GOP support in the Jewish community. Despite the fact that in every election cycle conservative Jewish activists claim that Jews are becoming more Republican, the Jewish vote for Republicans at the national level is stuck at about 20%. Moreover, over the last decade there are fewer and fewer Republican Jewish office holders at the national level.

With Specter joining the Democratic Party, he becomes the 12th member of the Senate that is Jewish and caucuses with the Democrats. When Al Franken, another Jewish Democrat, gets seated as the second Minnesota senator, there will be 13 Jewish senators who caucus with the Democrats in the Senate (compared to zero who caucus with the GOP). Democrats will then have 60 seats in the Senate, reducing the possibility of Republican filibusters.

I am not happy to have another right wing Democrat. But I AM happy to welcome Specter into the big tent that is the Democratic Party. I should note Pete McClosky, another high profile convert to the Democratic Party, suggested this was a possibility.