By Dean Goodman

LOS ANGELES (Reuters) - "The Dark Knight" fended off a strong challenge from the new "Mummy" sequel to lead the North American box office for a third weekend, and is on track to become the second-biggest movie of all time.

The Batman blockbuster earned $43.8 million for the three days beginning Friday, distributor Warner Bros. Pictures said on Sunday. Universal's "The Mummy: Tomb of the Dragon Emperor" followed with $42.5 million.

The Walt Disney Co. (DIS.N: Quote, Profile, Research) political comedy "Swing Vote" came in at No. 6 with just $6.3 million, the latest disappointment for its star, Kevin Costner, who has not had a $100 million movie since 1992's "The Bodyguard."

The total for "The Dark Knight" rose to $394.9 million. Warner Bros., a unit of Time Warner Inc (TWX.N: Quote, Profile, Research), said it expects the film to add at least $100 million, surpassing the $461 million haul of 1977's "Star Wars" and its two reissues, the current No. 2 movie of all time in the United States and Canada. The $601 million record, held by 1997's "Titanic," seems watertight.

"The Dark Knight" will add yet another record to its impressive tally on Monday or Tuesday, when it breaks $400 million, which would be its 18th or 19th day of release. The old record of 43 days is held by 2004's "Shrek 2."

MUMMY BIG OVERSEAS

Pundits had predicted that the "Mummy" movie could open to upwards of $50 million, roughly in between its two predecessors, but the critically mauled Brendan Fraser film was a little bruised by the Batman juggernaut.

It marks the follow-up to 2001's "The Mummy Returns," which opened to $68 million. The franchise relaunched in 1999 with a $43.4 million bow for "The Mummy."