Did you feel the earth move at Suffolk Downs? That happens when political ground shifts, and it just cratered at the East Boston racetrack.

Boom, boom, boom: Boston Mayor Thomas M. Menino — a big booster of casino plans for the site — isn’t running for reelection. Vornado Realty Trust — one of the casino’s largest stakeholders — just pulled out as an investor. Citywide support for a casino is soft, according to a new poll. Those surveyed also favor a citywide referendum, while casino backers want to limit the vote to Eastie residents only.

Even before Suffolk Downs absorbed those political tremors, its backers were coming to grips with serious and unexpected competition.

At the last minute, Las Vegas developer Steve Wynn came in with a proposal for a gambling resort in Everett, and Foxwoods Resort Casino joined forces with another plan to build a suburban casino in Milford.

Until recently, Suffolk Downs looked like the odds-on favorite to win a coveted go-ahead to operate a casino in the eastern Massachusetts region. Its backers had the mayor’s blessing, along with support from the rest of the local political establishment. A team of sophisticated consultants was gearing up to swat away a handful of anti-casino activists.

With no other contenders expected in the license hunt for the eastern Massachusetts region, the relatively low-profile activists were the only obstacle — or so Suffolk Downs’ backers believed. The plan was to shear off diehard opponents from more pragmatic ones who were open to making the best mitigation deal possible.

But never count your chickens or your casino licenses until they’re hatched. A license for Suffolk Downs is no longer a slam dunk — especially not with the stubborn and deep-pocketed Wynn in the mix.

The billionaire developer wanted to build a casino in Foxborough near Gillette Stadium. When the locals resisted, he didn’t take his dreams of blackjack tables and retreat to Vegas or Macau. He burst back onto the Bay State scene with a glamorous $1.2 billion proposal for humble Everett.

The property is odd-shaped, polluted, and presents traffic problems. But it has a view across the Mystic River to downtown Boston and support from Everett’s mayor. Like Suffolk Downs, Wynn needs an agreement with the host community and approval from voters. To accomplish that goal, he has his own team of sophisticated consultants working behind the scenes. He also has a reputation for being willing and able to write his own giant check to get what he wants.

The Suffolk Downs team of Richard Fields and Joe O’Donnell is partnered with Caesars Entertainment. Vornado Realty Trust, which had a 19.9 percent stake, pulled out after 11 representatives refused to file the financial disclosure forms required of all casino license applicants. According to a Globe report, Vornado’s shares were transferred to a trustee, who will submit to a background check; new owners of Vornado’s interest in the track will also have to submit to it.

During a Wednesday live chat on Boston.com, Chip Tuttle, the chief operating officer of Suffolk Downs, was asked about the impact of a mayor’s race on the casino proposal. “I don’t think it will have any impact,” he replied, adding that the timeline for the application process “should be completed before there’s a new mayor.”

But Menino’s decision triggers the start of a power vacuum at a critical time. The mayor was considered the not-so-secret weapon behind the application for an urban casino complex on a site that straddles Boston and Revere. When Wynn got involved, Menino made it clear he could make it difficult for him because some of the land needed for access to the Everett site belonged to Boston. However, the first rendering of Wynn’s proposal for the Mystic River waterfront does not rely on Boston land.

Suffolk Downs staked out early ground and has been working hard to reach agreements with Boston and Revere, the host communities. After that happens, a referendum must be scheduled for each municipality. The Boston vote will be citywide only if the Boston City Council votes to make it that way; a wide-open mayor’s race could influence the outcome.

As the latest political developments prove, when it comes to gambling, there’s no such thing as a sure bet.