Two senior House Democratic lawmakers are seeking new information from six top insurance companies in an ongoing probe of the industry’s business practices.

Energy and Commerce Chairman Henry Waxman (D-Calif.) and Rep. Bart Stupak (D-Mich.), chairman of that panel’s Subcommittee on Oversight and Investigations, on Monday asked the companies to turn over information about what they call their practice of “purging— small businesses from their rolls when covered employees get sick.

“We need to better understand how widespread this harmful and destructive practice has become, and how it is impacting small businesses and their employees across the country,— Waxman said in a statement.

The letters went to insurers Aetna, Humana, Medica, UnitedHealth Group, Wellmark Blue Cross Blue Shield, and WellPoint.

The missives are the latest in an ongoing assault by House Democrats on the insurance industry as the party seeks to rally support for its struggling health care overhaul plans. The letters come two weeks after Waxman and Stupak wrote to 52 insurers asking for details of the companies’ executive compensation packages, conferences and retreats they sponsored, and the profitability of their products. And last week, Rep. Dennis Kucinich (D-Ohio), chairman of the Oversight and Government Reform Subcommittee on Domestic Policy, requested that six top insurance executives ­­— including those for Aetna, Humana, UnitedHealth and WellPoint ­­— appear before his panel in September to testify on how they do business.

Waxman and Stupak are asking the companies to divulge their total number of policies covering groups of 100 people or fewer, the best-selling types, how they determine premiums for them, and explanations for the highest rate increases. The insurers have until Sept. 14 to produce the information, and until Sept. 8 to tell the committee whether they will voluntarily comply.