Sandwiched between two doctors’ offices at a roadside plaza here is the headquarters of a small team of veteran Republican investigators, operating almost as a private detective squad, who since late last year have had a determined goal: bringing down Senator Robert Menendez, Democrat of New Jersey…

To Mr. Menendez and his staff, the work going on at this suburban Washington office suite, paid for by donations from prominent Republicans nationwide, is proof that the still-building news media frenzy focusing on his actions to help a Florida eye doctor is at least in part a political smear…

But the way the investigation got started, at a minimum, illustrates the often-hidden role that partisan players have in helping push major news media outlets to dig into ethical allegations lodged against sitting members of Congress.

The inquiry began with an incendiary tip — unproven and vehemently disputed by Mr. Menendez — that Dr. Melgen had helped procure prostitutes, some of them underage, for Mr. Menendez, after flying the senator repeatedly on his private plane to the Dominican Republic, where Dr. Melgen has a home at a seaside resort. This information was put forward by an odd array of self-interested characters, including the right-leaning Web site The Daily Caller and someone — his identity remains a mystery — who claimed to be an American citizen who frequented the Dominican Republic.