Five-term Pennsylvania Republican Sen. Arlen Specter‘s decision, announced Tuesday, to change his party affiliation to Democratic for his 2010 re-election campaign is the latest setback for the GOP in the northeastern United States, where the rival Democrats have grown increasingly dominant in recent years.

While Specter did not immediately state when he plans to start sporting the Democratic Party label, it seems certain to be soon, given that he is now persona non grata among Senate Republicans.

When his switch becomes official, he will join Democrat Bob Casey in giving Pennsylvania two Democratic senators for the first time since 1946. That was the last full year that Democrats Joseph F. Guffey and Francis J. Myers served together.

In 40 of the 62 years since, Pennsylvania had two Republican senators.