He said the GOP will have to defend 19 seats, while Democrats have 17 seats to defend. Democrats this session have a majority in the Senate and House.
Republicans are retiring from Senate seats in five of the states: Ohio, Florida, Missouri, Kansas and New Hampshire.
Menendez contends the Democrats have an edge since Republicans have more seats to defend and it is difficult to unseat incumbents, The New York Times reported Tuesday.
"Four of those five states were carried by President Obama, and several by very significant margins," Menendez was quoted by the newspaper as saying.
He also said Democrats will be helped come Election Day because just three Republicans voted to support Obama's economic stimulus plan.
"The Republicans are in denial as to where the economy is and how desperate it is," he said. "They are in essence betting against the president and against an economic recovery."