HARRISBURG - Philadelphia will lose a state House seat while Chester County will gain one under a state legislative district map approved Monday.

In a 4-1 vote, the bipartisan commission charged with finalizing the legislative map signed off on census-driven changes to 203 House and 50 Senate districts.

The two legislative Republicans on the committee - House Majority Leader Mike Turzai and Senate Majority Leader Dominic Pileggi - supported the plan, as did House Minority Leader Frank Dermody and commission chairman Stephen J. McEwan Jr., president judge emeritus of Superior Court and a Republican former Delaware County district attorney.

Senate Minority Leader Jay Costa, who called the plan unconstitutional and tried to offer his own last minute Senate map, voted against it.

Anyone seeking to challenge the map has 30 days in which to appeal to the state Supreme Court.

Using data from the 2010 census, mapmakers redrew boundaries, shifting seats from areas with declining populations - namely the Southwest - to faster-growing areas in the Northeast.

While Philadelphia's population grew, it did not grow as fast as that of York County, which won the Northeast Philadelphia seat held by Republican Rep. Dennis O'Brien, who is retiring to take a seat on Philadelphia's City Council.

At least three municipalities in the Southeast were split under the plan drafted by Republicans.

West Chester, Haverford, and Upper Darby will now be represented by more than one House member.

Rep. Greg Vitali (D., Delaware), who represents Haverford Township, said he felt the map boundaries were driven by politics, not population.

"I'm disappointed that Haverford Township was split, certainly, but it wasn't unexpected," said Vitali. "Despite all the flowery words, this was about putting politics above what's best for the community."

Vitali said he believes Haverford was split to give Rep. Nick Micozzie (R., Delaware) more Republican voters in an increasingly Democratic county.

But House and Senate Republican leaders praised the process as more transparent than in earlier years and responsive to public comment.

"We've been committed to a fair, open, legal reapportionment process," said Turzai.

Pileggi said he worked with Senate Democrats to accommodate changes they sought and reduced the overall number of divided districts.

But Costa called it a "partisan plan" that violated the constitutional requirement not to split districts unless "absolutely necessary."

After the vote it still remained unclear how much the maps had changed because they were only distributed after the vote late Monday afternoon.

After the meeting ended, lawmakers and others were huddled around maps trying to figure out the impact to specific districts.

"The process has been going on for nine months and the public still has no real idea of what the plans contain," said Barry Kauffman, executive director of Common Cause of Pennsylvania.

Contact staff writer Amy Worden at 717-783-2584 or aworden@phillynews.com or follow on Twitter at @inqyamy.