In Nevada’s rough-and-tumble politics, Reid’s top backers include a heavy dose of the Nevada business community, especially in the casino and land development areas, and the telecommunications industry. Those business interests are balanced by sizeable support from organized labor and trial lawyers. ...
All told, Reid has raised more than $42 million over nearly three decades, including more than $6.5 million for the federal version of his leadership PAC, the Searchlight Leadership Fund. Representing a swing state like Nevada — in four of the last five presidential elections the state has been won by a margin of less than four points — Reid has needed almost every penny. The former middleweight boxer is currently in the political fight of his life, trailing in virtually every public poll when matched up against his likely November opponents. (Emphasis mine)
His top PAC donors over the years include AT&T, Laborers' International, American Federation of State, County and Municipal Employees, American Bankers Association, MGM Mirage and others. An interesting mix of business and labor groups, but that's unsurprising given that he leads the Senate and the federal government has enormous power and continues to take more of it.
Make sure to read their full report.