Maryland Gov. Martin O’Malley plans to announce as early as Thursday that he is calling lawmakers back to Annapolis next month to try to pass an expanded gambling plan that includes a Prince George’s County casino, aides said Wednesday.

The plan to be weighed in a special session is also expected to allow table games, such as blackjack and roulette, at Maryland’s five existing slots sites and provide some tax relief to those facilities affected by the additional competition in Prince George’s.

The new facility would most likely rise at National Harbor, the 300-acre mini-city on the banks of the Potomac River. Other locations nearby would also be permitted to bid.

“I believe the governor has made that determination to go forward,” House Speaker Michael E. Busch (D-Anne Arundel) told reporters after a private meeting with House Democratic leaders late Wednesday afternoon.

That assessment was confirmed afterward by several aides to O’Malley (D), who said the session will probably start between Aug. 8 and 13, and last a few days.

Any expansion legislation passed in the session would require voter approval in November.

Though an expanded gambling plan enjoys broad support in the Senate, it has been a tough sell in the House of Delegates. Rounding up the needed votes could still provide a challenge for O’Malley.

“I don’t know how the governor would know if the votes are there,” said Del. Curtis S. Anderson (D-Baltimore), who is the chairman of the city’s delegation in the House. “It’s a big gamble, pardon the pun.”

Aides to O’Malley have said in recent days that he is willing to risk failure in a special session in order to put the issue behind him for the remaining two years of his term.

Joseph C. Bryce, O’Malley’s chief lobbyist, said Wednesday night that a majority of delegates can be won over given the revenue at stake for the state and the jobs that would be created.

“The governor would not be doing it without the expectation that he gets the votes with the help of the speaker,” Bryce said.

Plans for a Prince George’s casino have collapsed twice in recent months, both times with resistance coming from the House. On the final night of this year’s regular legislative session, a Senate-approved bill stalled in the House amid brinksmanship between the two chambers over budget legislation.

A work group convened by O’Malley subsequently floated a similar plan that failed when three House members who participated withheld their support, citing a concern about cutting taxes on casino owners so soon after raising them on six-figure earners. The five O’Malley appointees and three senators on the panel supported the plan.

Since then, O’Malley — who stood on the sidelines during the regular-session debate, calling gambling “a distraction” — has redoubled efforts to pass a bill.

The issue has strained his relationship with Busch, who has shown limited enthusiasm for the issue, while helping his standing with Senate President Thomas V. Mike Miller Jr. (D-Calvert), one of the legislature’s chief gambling advocates.

Aides said the bill O’Malley will introduce will look much like the plan developed by his work group, with a few modifications.

Busch said Wednesday that he would like to see a provision giving Prince George’s voters a direct say over whether a casino comes to their county. Under current law, that issue is left to voters statewide.

The notion of a Prince George’s casino is vigorously opposed by the owner of Maryland Live!, the state’s largest casino, which opened last month in neighboring Anne Arundel County. Cordish, which built and operates the facility, argues that a National Harbor site would unfairly cut into its expected share of the Washington market.

“We continue to feel strongly that it is not in the best interests of Maryland, nor is it fair, to consider changing the rules pertaining to gambling in Maryland prior to the five casinos authorized in Maryland’s constitution being open and stabilized,” Joe Weinberg, managing partner of the company, said Wednesday.

Prince George’s County Executive Rushern L. Baker III (D) has been among the biggest supporters of bringing a casino to his jurisdiction. Baker, who opposed slots during his days as a delegate in Annapolis, continues to tout the expected share of revenue his county would get from the facility, as well as the hundreds of jobs he says it would create.

Baker’s championing of a casino has been criticized by some in Prince George’s, who argue that several social ills, including crime and addiction, would accompany a gambling venue. In 2007, when state lawmakers crafted Maryland’s slots program, a majority of delegates from the county opposed the idea and did not want to host a facility.

Penn National Gaming, the owner of Rosecroft Raceway, a harness track in Fort Washington, has also taken issue with Baker’s embrace of National Harbor.

“To the best of my ability and to the limits of our corporate resources, we will fight furiously to stop this rush to create an exclusive opportunity for National Harbor until we have assurances that Rosecroft will be part of the mix of any expanded gaming in Prince George’s County,” Peter M. Carlino, the company’s chairman and chief executive, wrote in a letter this week to O’Malley. A copy of that letter was obtained by The Washington Post.