Back in 2006, when she was leading the charge to wrest control of the House of Representatives from Republicans, Rep. Nancy Pelosi, D-Calif., got a lot of mileage out of claiming that a "culture of corruption" prevailed under the GOP. Today, Pelosi is Speaker of the House and a Democratic majority is in control — and the shoe is most definitely on the other foot.

In June, the House Ethics Committee grudgingly disclosed that it is investigating allegations that Democratic lawmakers exchanged appropriations for campaign contributions. "Pay to play" is the name for this cozy — and unlawful — arrangement that funnels cash to politicians in return for political favors.

It's an old story inside the Beltway, made more significant this time around for a variety of reasons. First, the money involved is not exactly chump change, even by Washington standards — $300 million funneled to a defense lobbying firm called PMA. Second, the firm is connected to a close ally of Speaker Pelosi, Rep. Jack Murtha, D-Pa., who had been her original choice for majority leader before Rep. Steny Hoyer got the job.

Finally and perhaps most important, it involves earmarks — the same dodgy practice that got a few Republicans in trouble back when they were in charge.

Allowing members of Congress to appropriate money for a special purpose or recipient — earmarking — may be benign in concept and useful in allocating public money for good projects. In practice, it has become an abuse of the public purse. Members of Congress, regardless of party, find it all too tempting to play fast and loose with public money via earmarks.

The PMA Group, the lobbying firm at the heart of this scandal, was started by a former aide to Murtha, who sits on the powerful appropriations committee. According to news reports, PMA or its clients gave some $40 million to members of Congress from 1998 to 2009. In 2008 alone, Congress set aside $300 million for PMA clients. Two other Democrats on the committee also have been linked to the scandal. Last November, federal agents raided PMA's offices, after which it went out of business.

All of this should have been enough for Speaker Pelosi to order an ethics committee investigation of the whole affair, but she dragged her feet until persistent scolding by Rep. Jeff Flake, R-Ariz., forced the issue. On June 3, the panel admitted that a probe was under way, but little has been disclosed since then.

The Democrats should have learned from the experience of the GOP: Trying to shield party colleagues from the embarrassment of an investigation usually backfires. If they have nothing to hide, there should be nothing to fear.

As for earmarks, the practice should be abolished. Find another way to use public money for public purposes. A better and more honest way.