Staying active in their circle, one former aide said, also requires that they help Mr. Baucus’s political career, through fund-raising and other assistance.

Several of the lobbyists regularly fly to Big Sky, Mont., for weekend fund-raising retreats that Mr. Baucus hosts, or attend more intimate events in Washington like a gathering last month near the Capitol, where Paul Wilkins, Mr. Baucus’s chief of staff, talked about the millions of dollars Mr. Baucus will need to raise for his re-election campaign next year.

Among them, the top givers include Jeffrey A. Forbes, Mr. Baucus’s former Finance Committee staff director, who has donated a total of at least $25,000 to Mr. Baucus, his political action committee or the Montana Democratic Party. He attended the retreat in February at the Big Sky resort, which included skiing, snowmobiling and a big family dinner at Buck’s T-4 Lodge. The totals grow much bigger — to hundreds of thousands of dollars — when donations from Mr. Forbes’s clients, including Verizon and Altria, and other partners at his lobbying firm, are counted.

Mr. Wilkins said that the donations and fund-raising had been vital, noting that the nearly $4.6 million expected in hand by the end of this month would rank Mr. Baucus’s campaign chest among the top 10 in the Senate.

“It allows us to scare off opponents,” Mr. Wilkins told the group, which included former Baucus aides turned lobbyists, at a Capitol Hill townhouse owned by Federal Express. “It is the basis of everything that we do. So thank you for your support and everything you have done for Senator Baucus.” A New York Times reporter in attendance was asked to leave the private event.

Asked later about his comments, Mr. Wilkins said, “There is no expectation that former staffers do anything related to the office. They are private individuals, if they want to donate fine. If they don’t want to donate, that is fine, too.”

Mr. Baucus’s office points out that the former aides who now work as tax lobbyists are a small fraction of those whose who have worked for him over the years. Still, several colleagues who have served more than two decades on the finance panel — including Orrin G. Hatch, Republican of Utah, and Charles E. Grassley, Republican of Iowa, both as ranking Republican or chairman — have much smaller networks of former aides who are tax lobbyists, according to the data collected by LegiStorm and the Center for Responsive Politics.