With Sens. Hillary Rodham Clinton and Barack Obama headed for a Pennsylvania showdown, their operatives are scrambling to get supporters who are independents and Republicans to register as Democrats so they can vote in the April 22 primary.

"One of our biggest supporters is Gov. Ed Rendell. We're going to be tapping his expertise in this [Pennsylvania] campaign to turn out independents, women and moderate Republicans," said Mark Nevins, Mrs. Clinton's communications director for Pennsylvania.

The Clinton campaign has held 42 events around the state, organizing volunteers and activists to reach out to new voters, especially women and blue-collar workers, Mr. Nevins said.

The Obama campaign is counting on an army of more than 2,000 volunteers, who have begun canvassing and making calls to potential new voters and Republicans, said Sean Smith, a spokesman for the campaign in Pennsylvania.

"We had over 400 people at a training event we held at our campaign office in East Liberty last Saturday," Mr. Smith said.

The registration deadline is March 24, according to registration guidelines issued by the Allegheny County Elections Division yesterday.

Voters can cast ballots only in the primary of the party with which they are registered. To vote for Mrs. Clinton or Mr. Obama, Republican and independent would need to change their registration by March 24.

With no contest in the Republican presidential race, many voters might do that.

"We have seen a bit of an increase in registration forms in the last few days, but I think we will see more and more [registrations] as the campaigns heat up," said Mark Wolosik, head of the county's Elections Division.

To vote, one must be 18 years old on or before April 22 and a United States citizen. Voters must be a resident of the election district they wish to vote in at least 30 days prior to the primary.

Mr. Wolosik said those wanting to register to vote may apply in person from 8:30 a.m. to 4:30 p.m. weekdays at the Office of the Registration Commission in Room 609 of the County Office Building, 542 Forbes Ave., Downtown.

The Elections Division will remain open until 5 p.m. March 24 to accept hand-delivered voter registration forms.

Registration-by-mail forms may be used to register or to change name, address or party affiliation. The forms are available at armed services recruitment centers, area agencies on aging, several branches of the U.S. Postal Service and state, county and municipal government offices.

Registration-by-mail forms must be postmarked no later than March 24. Forms also can be obtained by visiting the state Web site, www.dos.state.pa.us/voting.

Mrs. Clinton's campaign, currently in a temporary office, soon will open its Pittsburgh office on Smithfield Street, Downtown, Mr. Nevins said.

The campaign, which already has offices in Philadelphia and Harrisburg, also plans offices in Erie, Johnstown, Scranton, Allentown and State College.

With Mrs. Clinton revitalized by a big victory in Ohio, Mr. Nevins said Pennsylvania is next. "If we can get independents and moderate Republicans to cross over to us, that would be great. But our goal is to turn out our supporters and I think we will start with an inherent advantage here," he said.

With an office in East Liberty, Mr. Smith said the Obama campaign will appeal to a coalition of young and new voters, independents and Republicans.

"We feel great about the level of our grass-roots network in Pittsburgh and all across the state," he said. "We're going to have big support in Pennsylvania."

The campaign, which plans a total of 12 campaign offices across the state, has established operations in Philadelphia, Doylestown, Bethlehem and Harrisburg.