I never particularly liked Menendez. Maybe that's not so much a judgment on his person (though I find him unimpressive and somewhat swarmy) than it is a judgment on the political milieu he emerged from: the infamous Hudson County machine. Kean, his opponent, is not a bad substitute. He appears to be a chip off his old father's block (the moderate former governor, Tom Kean, chairman of the 9-11 Commission) and is running in a way that most Republicans in other parts of the country wouldn't recognize. If elected, there is a chance that he could fit into the Lincoln Chaffee mold.
Yet, that is not the full story. Somewhere in the interstices of conscience and constituency, there is room for raw party politics to play out. Having one of "your" guys in position may have benefits for the party on some crucial votes and as an overall orientation to government, which may in turn affect the political culture. So party affiliation can matter dramatically.
While I know the outline of Menendez's present difficulties, I don't know the day-to-day, blow-by-blow details. Is it possible he may step aside? Perhaps. (In my view, he was a purely "political" choice by Gov. Corzine to fill Sen. Corzine's seat. Menendez simply had a compelling demographic as a veteran Hispanic Congressman.) If he does, look for the New Jersey-patented candidate scramble, with most eyes, presumably, looking toward Bill Bradley. He would win. His popularity is great, and this is a very Democratic state. Short of a Bradley, however, the Democrats would appear to be in trouble. Kean is respectable, remember, and a challenger without instant celebrity may have an insurmountable task in undoing the good will he's generated in the normal time span of a regular campaign.