The credit card company filed a $5 million lawsuit Wednesday against Nader, accusing him of ripping off MasterCard's well-known "Priceless" advertising campaign in a new TV ad for the Green Party presidential candidate.

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Nader defended the ad Thursday, calling the lawsuit "foolish." He said MasterCard needs to "lighten up" and recognize that his ad is a legally acceptable parody.

"MasterCard is taking itself a little too seriously and, in typical corporate style, is trying not only to dominate the credit card industry ... but also wishes to control the arena of free speech and the free flow of creative ideas in the political arena," Nader said at a press conference.

MasterCard International Inc. feels otherwise.

"We feel we have one of the most successful advertising campaigns, period, and we will always do whatever it takes to protect this campaign," said Chris O'Neill, vice president of global marketing communications for Purchase, N.Y.-based MasterCard.

MasterCard filed its lawsuit in federal court in Manhattan, seeking an order barring Nader from running the ad. O'Neill said the company brought the suit after a week of fruitless conversations with the Nader campaign.

The Nader ad, which began airing Aug. 6 in San Francisco and Los Angeles, was scheduled to run only through Thursday. The campaign plans to continue to show the ad on its Web site indefinitely, Nader said.

MasterCard's ads feature sentimental episodes of families together at places including a beach or baseball game, assigning monetary values to various activities before coming up with an activity that is "priceless." The ads conclude: "There are some things in life money can't buy. For everything else, there's MasterCard."

The Nader ad adopts a nearly identical format to focus attention on the role of political contributions in this year's presidential campaign. The ad opens with video clips of Texas Gov. George W. Bush and Vice President Al Gore, as an announcer intones: "Grilled tenderloin for fund-raiser: $1,000 a plate. Campaign ads filled with half-truths: $10 million. Promises to special interest groups: over $10 billion."

"Finding out the truth: Priceless," the announcer concludes. "There are some things money can't buy. Without Ralph Nader in the presidential debates, the truth will come in last."

MasterCard sued HBO last year after the cable television network ran an ad for its comedy series "Arliss" that was mirrored the "Priceless" ads. That suit is still in litigation and HBO is no longer running the ad, MasterCard's O'Neill said.