Eighteen Democratic senators asked the General Accounting Office (search) on Thursday to investigate the Bush administration for keeping estimates of the cost of Medicare legislation from Congress.

The Health and Human Services Department inspector general already is investigating the matter, at the prompting of House Democrats and HHS Secretary Tommy Thompson, who said the inquiry is needed to remove "a cloud over this department."

As Democrats mount election-year attacks on the new prescription drug law (search), Senate Democratic leader Tom Daschle and his colleagues said GAO should examine whether the administration violated a law shielding federal employees from political pressure. GAO is the investigative arm of Congress.

Thompson has denied playing any role in preventing Richard Foster (search), the Medicare actuary, from sharing his higher estimates of the legislation's cost with Congress. Thomas Scully, who ran the Medicare agency until December, threatened to fire Foster if he gave Democrats his analysis of the legislation, The Associated Press reported in June.

Since President Bush signed the Medicare (search) prescription drug law in December, the administration has acknowledged it believes the law will cost $534 billion over 10 years, compared to the $395 billion estimated by congressional budget analysts.

Keeping the bill's cost below $400 billion was considered critical to attracting the votes of conservative Republican lawmakers, some of whom vowed to oppose more expensive legislation.