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Winslow Townson, File, Associated Press
In this Jan. 9, 2015, file photo, Boston Mayor Martin Walsh speaks during a news conference in Boston after the city was picked by the USOC as its bid city for the 2024 Olympic Summer Games. Walsh said Monday, July 27, 2015, he won't sign a host city contract, which is key to the city's bid, without more assurances that taxpayers won't foot the bill.

Boston's mayor delivered a harsh blow to the city's effort to host the 2024 Olympics on Monday when he declared he wouldn't sign any document "that puts one dollar of taxpayer money on the line for one penny of overruns on the Olympics."

That document is the host city contract that most in the Olympics consider crucial to any city's success. Walsh made his announcement at a hastily called news conference that coincided with Gov. Charlie Baker's meeting with the U.S. Olympic Committee.

The USOC was pressing Baker for assurances that he would back the bid, but Baker said he couldn't do it until he saw a detailed report about the games from a consulting group. That report isn't expected until next month.

Olympic bids need strong support from key city leaders. The USOC board was meeting later Monday to discuss the future of the bid. Baker's recalcitrance, combined with an about-face from a previously supportive Walsh, gives the board an opening to pull the plug on Boston, where public support has lagged.

Walsh didn't say the bid was dead by his estimation, but he hinted of no plans to do what's being asked of him. The host city contract is often a difficult sticking point for U.S. candidates, because unlike other countries, the federal government doesn't back any Olympic efforts. It doesn't need to be signed until 2017, but the USOC is eager to have it delivered well before then.

Walsh said it won't happen until he knows the taxpayers won't have to foot the bill.

"If committing to signing the guarantee today is what's required to move forward, then Boston is no longer pursuing the 2024 Olympic and Paralympic games," he said.

The official deadline to nominate a city isn't until Sept. 15, but the USOC is looking for answers right away. That's in part because there still could be time to get Los Angeles on board, and because there's a key International Olympic Committee meeting taking place later this week in Malaysia, where the 2024 bid will be a hot topic. Leaders of the Boston bid were expected to attend.

They've been under pressure to shore up poll numbers that show public support in the 40 percent range.

Plans are being made for a voter referendum next year, and the bid leaders have promised if it doesn't win both in Boston and statewide, they would pull the bid.