DISCLAIMER: This whole scenario is unlikely, but is a distinct possibility.
Bob Menendez has gotten himself into a real mess. It is never a good idea to allegedly go cavorting around the Dominican Republic with underage prostitutes on a trip paid for by a wealthy eye surgeon who is also a campaign donor. The Senate Ethics Committee is currently investigating the ethics violations, and if the FBI determines that the prostitutes were underage then Menendez will likely resign. Sleeping with under-18 year olds is too much for even New Jerseyans who are used to corruption among their elected officials. Should Menendez resign in the coming months, Chris Christie would be empowered to appoint a temporary Senator and schedule a 2013 (or 2014, if Menendez resigns later in the year) special election. This special election would give the GOP a good chance of keeping the seat- quasi-incumbency granted to the appointee would be an advantage, and if the GOP wins then that seat stays in GOP hands until the 2018 election, which would either be the first midterm for the new Republican president (making it a fight to keep it), or the first midterm for the new Democratic president (making it easier to keep, as by that time 10 years of Democratic rule will be 10 years too many). Either way, the GOP hanging onto a seat in deep-blue New Jersey would be a huge coup.
Christie’s popularity would be a huge boon to the GOP appointee. The GOP has a decent bench in New Jersey who would be able to capitalize quickly on a temporary Senate appointment. From Congress, there is Frank LoBiondo (NJ-02), Jon Runyan (NJ-03), Chris Smith (NJ-04), Scott Garrett (NJ-05), Leonard Lance (NJ-07), or even Rodney Frelinghuysen (NJ-11). Known statewide are Lt. Governor Kim Guadagno, who would be a strong choice, as would former Senate nominees Tom Kean, Jr (2006) or Joe Kyrillos (2012). I also heard tell that New York Jets owner Woody Johnson would be in the running for the appointment, and his money would be an asset in keeping the seat. Unfortunately whoever this hypothetical appointee would be would probably be more moderate than I would prefer, but I would be willing to live with that if it means making the gap closer in the Senate and making it easier to take back the Senate in 2014.
On the left side, Cory Booker would be unlikely to run in a special- Lautenberg still looks likely to retire and it would be easier for him, so disliked by the Democratic establishment in New Jersey, to run for an open seat than against an incumbent. Aside from Booker, there are Rob Andrews, Frank Pallone, Sheila Oliver, Richard Codey, Bill Pascrell, or Stephen Sweeney, who all run close races in polling against Lt. Gov. Guadagno. New Jersey will be a long shot, but it is certainly a possibility.
As a side note, should Lautenberg pass away in the next 5 years, this whole game would start again, and if Booker implodes, Christie stays popular, and the Republican candidate is well-funded and smart, there could well be 2 Republican Senators from New Jersey. Its a slim chance, but anything could happen in politics.