WASHINGTON — The Justice Department will not try to block the acquisition of the travel booking site Orbitz by its larger rival Expedia, clearing the way for the companies to complete the $1.3 billion merger announced earlier this year.

Hotel companies, along with some consumer advocates and lawmakers, raised concerns over the potential market power of the combined company, which would control roughly 75 percent of the domestic market for third-party online booking, according to the research firm Phocuswright.

“We took those concerns seriously and factored into our analysis all of the information provided,” Bill Baer, an assistant attorney general in the Justice Department’s antitrust division, said in a statement on Wednesday. “At the end of this process, however, we concluded that the acquisition is unlikely to harm competition and consumers.”

Mr. Baer said the Justice Department determined that Orbitz was by comparison a small source of bookings for many hotel companies, and that the business of online travel was “evolving rapidly.” He cited offerings by TripAdvisor and Google that aim to move beyond just searching for rooms to booking them also.

Expedia, in a statement, said it was pleased with the decision: “We look forward to officially welcoming Orbitz Worldwide.”

But opponents in Washington pledged increased scrutiny of online booking sites.

“I continue to be concerned that this merger will harm consumers and travel providers by restricting competition,” said Senator Amy Klobuchar of Minnesota, the top Democrat on the Senate antitrust subcommittee.

William A. Crow, of Raymond James & Associates, said that while the commissions that online-booking sites charge hotels were down from their post-9/11-era heights, they remained elevated. “This may prompt the hotel brands to market their own websites even more aggressively,” he said, or enter less pricey deals with sites like TripAdvisor.