A slot machine at the Hollywood Casino Perryville in Cecil County. (Photo by Doug Kapustin/For The Washington Post)

The additional share of revenue would be intended in part to compensate casino owners for the new competition that would come with a new venue in Prince George’s County.

The legislation, which O’Malley could release as early as Tuesday afternoon, is expected to call for public votes on both a Prince George’s casino and allowing Las Vegas-style table games at all Maryland slots venues. A special legislative session is set to begin Thursday.

The marketing allowance idea comes from New York, which last year gave casino operators, on average, an additional 8.7 percent of slots revenue to use for advertising and promoting their facilities. That allowance was on top of an average share of 33.5 percent that casinos received without restrictions.

In Maryland, most operators are allowed to retain 33 percent of slots revenue, with much of the rest going to the state, host counties and horse-racing industry.

It was not immediately clear how much of a marketing allowance Maryland might offer. But people familiar with the governor’s bill, which remained a work in progress Tuesday, pointed to the increases that were included in a plan developed this spring by a work group O’Malley formed.

That plan did not use the term “marketing allowance” but instead increased the current 33 percent share at casinos that would be most affected by new competition from Prince George’s.

Under the work group plan, casinos in Anne Arundel County and Baltimore would get an automatic 5 percent increase once a Prince George’s facility opened and could petition for a larger increase from the state. Three other casinos would not get an automatic increase but could make the case to the state that they merit an upward adjustment.