STROUDSBURG, Pa. - Slowly but surely, Pennsylvania is tilting southeastward.

As the population shrinks in western Pennsylvania and grows in eastern Pennsylvania, the politically pivotal state is becoming more suburban, more Democratic, more eastern. It is becoming more like New Jersey and less like Ohio.

Since 1970, the 16 westernmost counties have lost about 400,000 people, according to census data. Meanwhile, the 15 easternmost and southeasternmost counties have gained about 900,000.

Here in Monroe County, in the heart of the Poconos, the population grew by nearly 30,000 - 20 percent - between 2000 and 2006 as former residents of New Jersey and New York continued to move in, attracted by lower housing costs and lower taxes. To the north, neighboring Pike County gained 26 percent, and in adjacent Northampton County, to the south, population was up 9 percent in the same period.

"We're finally getting some of the restaurants we always wanted, and now we don't have to go to Scranton or Wilkes-Barre or Allentown to shop," said Suzanne McCool, a retired New Jersey teacher who is the new chairwoman of the Monroe County commissioners. "Monroe County used to be very rural and conservative. With the influx of people, they've given us a new vitality. . . . Diversity is up. We're like a small city in the makeup of our county."

The gains in the south and east, despite big population losses in Philadelphia, have increased the influence of the state's younger, more affluent, more urban residents. Politically, the shift has made the state less conservative, though it remains less liberal than New Jersey and New York.

Even in Chester County, a Republican bastion, Democratic registrations are creeping up and GOP registrations slipping, though Republicans still hold a registration edge of 48 percent to 37 percent.

"I moved here from western New York because of greater economic opportunity," said Tom Curtin, who lives in Parkesburg and works for Independence Blue Cross in Valley Forge and Philadelphia. "I think, overall, people in the region are becoming less conservative than they were in the past."

"I know it's been said that the Republicans in Chester County are more liberal than the Democrats outside of Pittsburgh," said Curtin, a Democratic councilman in Parkesburg.

Lorraine Stanish, a retired nurse and insurance worker who lives in East Marlborough Township, said that "Democrats are coming out of the closet" in the county.

Stanish, who moved to Chester County 30 years ago from South Philadelphia, said she thought "many people had Democratic views but weren't out in the open because there had been such a heavy Republican presence."

"We have a younger population coming in," said Ann McHale, a Democrat who is the new president of the Northampton County Council. "I see them with a more modern, realistic view of things, more of a liberal attitude. They're less conservative than the older folks here, which is a good thing."

The thousands of new residents streaming into Pennsylvania's border counties often continue to commute to work in another state.

From Monroe County, "we have 22,000 people a day who commute to New Jersey and New York," said Robert Phillips, chief executive officer of the Greater Pocono Chamber of Commerce. That is 20 percent of the county's working-age population.

About 60 New York-bound buses a day roll out of the depots in Mount Pocono and Delaware Water Gap, starting at 4 a.m. The trip takes nearly two hours, each way, and a monthly ticket costs about $450.

"It's a tough life, but people are willing to make the sacrifice," Phillips said. "For so many people, they find an oasis here. . . . It's a great place to raise a family."

To help cope with the swarms of commuters, Pennsylvania and New Jersey are working to restore the old rail line between Scranton and New York City that was abandoned and ripped up in the 1970s.

Likewise, Pennsylvania's southern border has been discovered by workers in the Baltimore and Washington regions.

Adams County's population swelled by 10,000 - 11 percent - between 2000 and 2006. Neighboring Franklin County grew 8 percent, and York County added about 35,000 people, or 9 percent.

"They're definitely coming from the Maryland and D.C. area," said Adams County Commissioners Chairman George Weikert, a Republican. "There are housing moratoriums in some parts of Maryland, so developers are coming here and buying land up here."

"It certainly is taking a toll on our agricultural land," Weikert said, "and the services that are needed are putting a strain on county government."

The separation between new and old Pennsylvania runs roughly from Franklin County in the central south to Wayne County in the northeast, tracing a cultural and demographic divide as pronounced as the geographical boundary provided by the Appalachian Mountains.

"Demographically, the two parts of the state are very different," said Leif Jensen, a professor of rural sociology and demography at Pennsylvania State University. "In the west, the youth are moving out and leaving behind an aging population. . . . In the east, you have the stresses of the population increase."

"Clearly, the movement is going to make the southeast more powerful," Jensen said. "The seesaw tips ever more in that direction."

The shift is apparent in the gradual movement east of the state's center of population. In 1940, Pennsylvania's population center was in Juniata County, near the little town of Tuscarora. By the 2000 census, the center had moved about 15 miles east, across the Tuscarora Mountains, to a point near New Buffalo in eastern Perry County.

"Pennsylvania has always had a dual personality," said James Cowhey, executive director of the planning commission of Lancaster County, where the population has grown by 56 percent since 1970. "When you get on the other side of Cumberland County, you can sense the Midwestern nature of the place.

"There's always been that dual personality, but the state used to be in much better balance economically. That's not the case now."

Of the 19 counties with a median household income of more than $45,000, only one - Butler County - is in Western Pennsylvania, according to census data.

And the effect can be seen on voter registration.

New arrivals from urban areas "are going to bring with them more cosmopolitan attitudes," said Jensen, the Penn State demographer. "What are the implications of that in voting for one party or another?"

In the last 10 years, the percentage of Democratic voters has increased in most eastern and southern counties, while the percentage of Republican voters has declined. Statewide, Democrats now hold an advantage of 50 percent to 39 percent, up from 48 percent to 42 percent in 1998.

"I thought this was a Republican county," said McCool, who was the lone Democrat among the three Monroe County commissioners until November's election. Now, with two Democratic commissioners for the first time in 20 years, McCool is chairwoman. "Now there are about 8,700 more Democrats registered than Republicans."

But just as people can change the fabric of a community, so can a community change the attitudes of people.

Famously conservative Lancaster County has grown by about 179,000 people since 1970. But it remains overwhelmingly Republican: 57 percent to 29 percent for Democrats.

"The culture is strongly rooted in the county," planner Cowhey said. "It's a firmly rooted culture that tends to change people; it makes them more conservative. On the other hand, I have seen some pushback from people who have moved here."

Contact staff writer Paul Nussbaum at 215-854-4587 or pnussbaum@phillynews.com.