Robert Menendez, the once-powerful U.S. senator from New Jersey who was convicted last year of participating in a brazen scheme to trade political clout for bribes, could spend much of the rest of his life in prison after he is sentenced on Wednesday in Manhattan.

Federal prosecutors, who have described Mr. Menendez’s conduct as possibly “the most serious for which a U.S. senator has been convicted in the history of the republic,” have asked Judge Sidney H. Stein of Federal District Court to impose a sentence of at least 15 years in prison.

Lawyers for Mr. Menendez, 71, citing his hardscrabble upbringing, life of service and devotion to his family, are seeking a much shorter term, of no more than 27 months, with “at least two years’ rigorous community service.”

The lawyers, Avi Weitzman and Adam Fee, also asked the judge to consider whether sparing him prison and instead sentencing him to home detention with the community service provision would best serve “the ends of justice in this case.”

Mr. Menendez has maintained his innocence and plans to appeal the verdict. A jury found him guilty on all 16 counts he faced, including bribery, extortion, honest services wire fraud, obstruction of justice, conspiracy and acting as an agent for Egypt.

He is the first U.S. senator to be convicted of acting as an agent of a foreign power and the seventh senator convicted of a federal crime while in office. He resigned in August.