The figures, which showed modest declines in the ranks of Republicans and independents, reflected intense interest in the race for the Democratic presidential nomination and recruitment efforts by both candidates, Sens.of New York and of Illinois.
Since last year's election, the number of Democrats increased by more than 161,000 - more than 4 percent - to at least 4,044,952. No political party in the state had previously reached the 4 million threshold. Registration in the GOP shrank by about 1 percent to 3,215,478 statewide.
The figures released by state elections officials did not include the final hours of voter registration in the state's 67 counties or mailed-in applications, which will count as long as they are postmarked Monday.
The largest percentage gains were concentrated in the Philadelphia suburbs and the central region of the state, mostly in counties where Republicans still outnumber Democrats.
Of the more than 8.2 million Pennsylvania voters, more than 120,000 are people who were not previously registered to vote.
Pennsylvania's electorate is distributed fairly equally among age groups, the state statistics show. Nine percent are between 18 and 24; 16 percent are between 25 and 34; 18 percent are 35-44; 20 percent are 45-54; 16 percent are 55-64; 10 percent are 65-74; and 11 percent are 75 or older.
The biggest age group for both Democrats and Republicans is 45-54. The proportion of voters 18-24 is 9 percent for Democrats and 7 percent for Republicans.
With four weeks remaining until the April 22 primary, Clinton retains a strong lead over Obama in Pennsylvania. A Quinnipiac University poll earlier this month showed Clinton favored by 53 percent of likely Democratic voters, and Obama by 41 percent.
The business of registering Pennsylvanians to vote was brisker than ever in many counties on the last day for registering.
"It's very heavy. We had people at the counter long before we opened up" Monday morning, said Joseph Passarella, director of voter services in Montgomery County.
Montgomery County is among the suburban Philadelphia counties that have seen large increases in Democratic enrollment as the nomination battle rages on.
Check out the upcoming primary and caucus dates.
The county office stayed open Saturday to accommodate late registrants in a year that the protracted campaign has made Pennsylvania's primary more important than usual and piqued the interest of voters who have stayed outside of the major parties or not bothered to register at all.
Only registered Democrats and Republicans can vote in the primary, which also includes candidates for nominations for the statewide row offices, Congress and the state Legislature.
At the Luzerne County elections office in Wilkes-Barre, Director Leonard Piazza III said the pace was "very brisk" Monday, including hundreds of fresh applications dropped off by the Clinton and Obama campaigns.
"This is more like what we see in November" for general elections, he said.
In Cumberland County, whose elections office in Carlisle was closed for Good Friday, office Director Penny Brown said Monday was very busy.