On the topic of his party’s chances in next year’s elections, Robert Menendez is a study in resolute optimism.

Not that he has much choice. Mr. Menendez, New Jersey’s junior senator, chairs the Democratic Senatorial Campaign Committee, which recruits and provides financial and political support for the party’s U.S. Senate candidates. Optimism is part of the job.

But in 2010, Mr. Menendez will have to deal with some unpleasant realities that his predecessor, the irrepressible Chuck…