LOS ANGELES (Reuters) - MGM Resorts International won its costliest political gamble to date after Maryland voters passed a divisive gambling expansion ballot that enables the casino giant to move forward with a bid to build an $800 million resort casino in the state.

Approval on Tuesday of the ballot measure, known as Maryland Question 7, will permit bidding for a sixth casino in the state and is expected to pave the way for a new MGM casino at National Harbor in Prince George's County, which it says will create 2,000 construction jobs and up to 4,000 permanent jobs.

National Harbor is located on Potomac River, just minutes from Washington D.C.

MGM paid the majority of the $45.3 million spent by a consortium including Peterson Development Cos LLC and local unions that backed the measure, against local casino owner Penn National Gaming Inc. Both sides anteed up a combined $90 million dollars on the ballot, in what Maryland officials said was a record amount spent on any one initiative.

"The people of Maryland have delivered a clear message," said MGM Resorts Chief Executive Jim Murren, who in a recent interview described the amount spent as "epic."

Penn National led the opposition and spent $42 million to block its passage in a bid to protect its Rosecroft Raceway facility in Fort Washington, Maryland.

Penn has long said the raceway will not be seriously considered as a potential site for a sixth casino license against MGM. It bought the raceway for $10.25 million in a bankruptcy auction in 2011 in hopes of revitalizing it as a casino.

Penn National also operates the Hollywood Casino at Charles Town Races in neighboring West Virginia, and the Hollywood Casino in Perryville, Maryland.

Penn has long complained that its prospects were scuttled by a "back-room" deal involving MGM and officials of National Harbor. MGM already has an agreement with developer Peterson Cos to build the casino in National Harbor.

"We are obviously disappointed. We appreciate all those who stood up against this unseemly back-room deal with National Harbor ..." Penn National said in a statement on Wednesday.

The measure sets the stage for Maryland's largest gambling expansion since 2008, when voters approved a ballot measure for slot machines at five locations.

If MGM wins the bid, as is widely expected, the new casino would open in 2016. "We think we'll win, but any number of companies can bid," Murren told Reuters recently.

Shares of MGM Resorts fell 2 percent to $10.11 a share, while Penn was off 1 percent to $39.60 a share.

Analyst Bill Lerner of Union Gaming called the vote a negative for Penn and a positive for MGM, saying a new MGM casino in National Harbor will hurt Penn's West Virginia casino.

"While the (MGM) casino wouldn't open until 2016 we think the impact to Charles Town in today's dollars is worth roughly $2.50 to $3 per share," he said, referring to Penn shares.

Spending by both sides on this issue hit a record, according to Maryland officials.

"Spending on this one ballot alone totaled 60 percent of the entire Maryland gubernatorial race in 2006. The amounts spent on this election have never been seen in the history of Maryland. You had a lot of competing interests with very deep pockets," said Jared DeMarinis, director of candidacy and campaign finance for the Maryland State Board of Elections.

By comparison, spending on the four-year Maryland gubernatorial election cycle ended December 31, 2006, which covered several candidates and ballots, was about $155 million, he said.

(Reporting by Susan Zeidler; Edited by Ronald Grover and Jeffrey Benkoe)