WASHINGTON -- For a few moments last week, a routine Senate hearing on aviation security took on the atmosphere of a courthouse trial in which a tough prosecutor grilled an uncooperative witness.

Leading the cross-examination was Missouri Sen. Claire McCaskill. In the hot seat was Peggy Gilligan, the Transportation Department's deputy associate administrator for aviation safety.

McCaskill, a freshman Democrat, kept pressing until Gilligan conceded a key point -- that U.S. taxpayers are footing the bill for extra security while airlines take advantage of lower labor costs at foreign repair stations.

When McCaskill was done, veteran Sen. Frank Lautenberg, D-N.J., passed a note over to the subcommittee chairman, Sen. Jay Rockefeller, D-V.W., with one word about McCaskill's performance: "Wow!!"

Since being sworn in nearly six months ago, McCaskill has impressed colleagues with her aggressive style and resolute focus on curbing government waste and abuse.

That style persuaded Rockefeller to let McCaskill lead the recent subcommittee hearing on gaps in aviation security, a rarity for a lawmaker in office less than a year.

"I turned it over to her as quickly as I could, and then she just took off, and she was fantastic," Rockefeller said. "She's totally unintimidated about being in her first term here."

The former prosecutor and Missouri state auditor has used her experience in financial oversight to shine a light on fraud and mismanagement in defense contracts and security lapses at aviation repair facilities overseas.

"It's probably going to be the focus of my time as senator, on financial issues as it relates to cost savings and being very stingy with the taxpayer dime," McCaskill said.

McCaskill was not known to be shy during her eight years as state auditor or as a member of the Missouri Legislature. And in the Senate, where it takes time to emerge as a leader, she already is ahead of the curve.

Two months into her term, she was the first lawmaker to call for the resignation of Lt. Gen. Kevin Kiley, the Army's top medical officer who was blamed for problems leading to the scandal at Walter Reed Army Hospital.

McCaskill also has worked with Republicans, when she can find common ground.

Earlier this year, she and Sen. John McCain, R-Ariz., offered an amendment to disclose all the earmarks during committee consideration of the annual defense authorization bill. All but one of her Democratic colleagues on the Senate Armed Services Committee were against the measure, but it prevailed.

"She seems to get along well," said Missouri's senior senator, Republican Kit Bond. "I see her always engaged on the floor with her colleagues on both sides of the aisle."

But McCaskill's stance against the traditional secrecy surrounding earmarks has pitted her against many in her own party as well as Bond, who is known for bringing the bacon home to his constituents.

McCaskill says she won't insert any earmarks this year for Missouri projects, and she supports proposals to require greater transparency in the process so the public can see exactly how tax money is being spent.