"If you're expanding the market to include any entertainment option, Las Vegas concentration becomes less of a concern," said Sanjay Ayer, an equity analyst at Morningstar. "I think that's these companies' defense." (Ayer owns no shares in companies he covers, while Morningstar neither does nor seeks to do business with companies its analysts cover.)

MGM Mirage spokesman Alan Feldman said the company now competes in a "global marketplace" for leisure dollars. He contends that even if regulators take a narrower view of Las Vegas, the market there alone remains hyper-competitive, even with a combined MGM Mirage/Mandalay. New players continue to jump in, he said, noting Landry's Restaurants ( LNY) recent plans to purchase the Golden Nugget in downtown Vegas. He also pointed out that all casino operators will face stiff competition from Wynn Resorts' ( WYNN) lavish Las Vegas casino, which is set to open in April.

An FTC spokeswoman said the commission doesn't comment on open investigations.

If the FTC weighs in positively on MGM Mirage, it's also likely to approve the Harrah's deal, given Harrah's smaller number of Las Vegas properties, analysts said.

State regulators will likely take their cue from the federal government. "In New Jersey and Nevada, I don't think there's too much of an issue after the FTC signs off," said Matt Quinn, senior lodging and gaming analyst at Zack's Investment Research. (Zack's neither does nor seeks to do business with companies it covers, and Quinn owns no shares of companies he covers.)

Spokespeople for Nevada's gaming regulators declined to comment.

Observers were not sanguine about the Harrah's merger last year. Harrah's and Caesars needed to rid themselves of properties in several states to comply with local regulations. Analysts were skeptical the companies would be able to quickly sell off casinos at good prices. But the companies assuaged most of those worries when they announced a deal to sell four of their casinos to Colony Capital LLC for $1.24 billion.

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