Baltimore's leaders have long sought ways to preserve the city's influence in Annapolis and Washington as its population declined.
The number of city residents peaked at nearly 950,000 in 1950 and has dropped every decade since, even as the population of the state has grown.
Recent figures show that the city lost about 14,000 residents between 2000 and 2008 and had a population of about 637,000.
Since 1974, each statewide reapportionment has cost the city at least one Senate district.
As recently as 2002, 10 of Maryland's 47 state senators represented all or part of Baltimore.
In that year, Maryland's highest court threw out a plan that included Senate districts that crossed the city-county line, a redistricting plan created by then-Gov. Parris N. Glendening to preserve Baltimore's influence. That cost the city four of its senators.
Currently, six state senators represent Baltimore, and the inclusion of inmates could provide a buffer against a further loss.
A state Senate district will contain about 120,000 residents after the next redistricting, up from about 112,000 now.
Other states are considering population rules similar to the one approved in Maryland.
In New York, some local lawmakers are pushing a plan to exclude prisoners from reapportionment drawings entirely.
"They play no part in our community," said Edward P. Welsh, a Republican county legislator from Utica. "They can't vote, they can't take part in the community, and I'm assuming they don't want to be here - so why are they being counted here?"
Some states are using the new census information differently.
Kansas officials want to remove college students and military personnel from redistricting calculations.
In Somerset County on Maryland's Eastern Shore, officials hope the new rule leads to the election of the county's first-ever black county commissioner.
A prison built in the 1980s, they said, disrupted a settlement of a Voting Rights Act lawsuit intended to create a majority-minority county legislative district.
Though the county is 40 percent black, no black representative has ever been elected to office.
"It's been a long time coming. I believe that people in the county, both black and white, are ready for change," said Clarence Bell, who was the county's first black police chief.
Lawmakers said they were pleased that the new law could bring diversity to places like Somerset.
"I am so proud that Maryland did this, I think that it speaks highly of us," said Del. Joseline Pena-Melnyk, a Prince George's County Democrat who was a lead sponsor of the House legislation.
"It's really a civil rights issue, a fairness issue and an equality issue - plain and simple."
Baltimore Sun researcher Paul McCardell contributed to this article.
Prisoner tally set to boost city data
First-of-its-kind Md. law could add as many as 12,000 to Baltimore's population
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