You better watch out. There is a new combatant in the Christmas wars.

Ads proclaiming, “Why believe in a god? Just be good for goodness’ sake,” will appear on Washington, D.C., buses through December. The American Humanist Association unveiled the provocative $40,000 holiday ad campaign earlier this month.

In lifting lyrics from “Santa Claus is Coming to Town,” the Washington-based group is wading into what has become a perennial debate over commercialism, religion in the public square and the meaning of Christmas.

“We are trying to reach our audience, and sometimes in order to reach an audience, everybody has to hear you,” said Fred Edwords, spokesman for the humanist group. “Our reason for doing it during the holidays is there are an awful lot of agnostics, atheists and other types of non-theists who feel a little alone during the holidays because of its association with traditional religion.”

To that end, the ads and posters will include a link to a Web site that will seek to connect and organize like-minded thinkers in the D.C. area, Edwords said.

There was no debate at the Washington Metropolitan Area Transit Authority over whether to take the ad. Spokeswoman Lisa Farbstein said the agency accepts ads that aren’t obscene or pornographic.

Edwords said the purpose isn’t to argue that God doesn’t exist or change minds about a deity, although “we are trying to plant a seed of rational thought and critical thinking and questioning in people’s minds.”

The group defines humanism as “a progressive philosophy of life that, without theism, affirms our responsibility to lead ethical lives of value to self and humanity.”

Last month, the British Humanist Association caused a ruckus announcing a similar campaign on London buses with the message: “There’s probably no God. Now stop worrying and enjoy your life.”

In Washington, the humanists’ campaign comes as conservative Christian groups gear up their efforts to keep Christ in Christmas. In the past five years, groups such as the American Family Association and the Catholic League have criticized or threatened boycotts of retailers who use generic “holiday” greetings.

In mid-October, the American Family Association started selling buttons that say “It’s OK to say Merry Christmas.” The humanists’ entry into the marketplace of ideas did not impress AFA president Tim Wildmon.

“It’s a stupid ad,” he said. “How do we define ‘good’ if we don’t believe in God? God in his word, the Bible, tells us what’s good and bad and right and wrong. If we are each ourselves defining what’s good, it’s going to be a crazy world.”

Also on Tuesday, the Orlando, Fla.-based Liberty Counsel, a conservative Christian legal group, launched its sixth annual “Friend or Foe Christmas Campaign.” Liberty Counsel has intervened in disputes over nativity scenes and government bans on Christmas decorations, among other things.

“It’s the ultimate grinch to say there is no God at a time when millions of people around the world celebrate the birth of Christ,” said Mathew Staver, the group’s chairman and dean of the Liberty University School of Law. “Certainly, they have the right to believe what they want but this is insulting.”

Best-selling books by authors such as Richard Dawkins and Christopher Hitchens have fueled interest in “the new atheism” — a more in-your-face argument against God’s existence.