Volunteers from President Obama's political organization are knocking on doors across Pennsylvania and the nation Saturday to urge people who voted for the first time in the 2008 presidential election to turn out this fall for Democratic congressional candidates.

Organizing for America, which grew from the grassroots network Obama built two years ago and is a part of the Democratic National Committee, says that up to 16,000 volunteers are expected at more than 1,300 canvassing and phone-bank events nationwide. Thirty-six events are planned in Pennsylvania.

Energizing the 15 million voters who cast ballots for the first time in 2008 – a majority of whom supported Obama, exit polls suggest – is a key component of the Democratic Party's plan to hang onto majorities in the House and Senate.

Party officials say that these voters are vital to try to blunt Republican momentum as public opinion has shifted against some of the Democrats' agenda amid continued worry over the economy and federal spending. The party in control of the White House almost always loses congressional seats in the first midterm election, and voter turnout declines from presidential years.

"We want to change the paradigm of midterm elections," said Greg Meyers, 55, OFA's lead community organizer for Chester County.

Locally, OFA volunteers will fan out after a 10:30 a.m. Saturday rally in Phoenixville (at Bridge and Main streets). Senate nominee Joe Sestak plans to attend, as does Manan Trivedi, the party's nominee for the 6th District House seat.

Elizabeth Lucas, state director of OFA, said that there were 729,000 first-time voters in Pennsylvania. For example, if the group were to generate an 8 percent increase in turnout among them over normal levels, it would produce 58,000 extra voters, Lucas said - equal to 2 percent of the 2006 midterm turnout statewide.

"We can make a difference on the margins and close races are decided on the margins," she said.

Contact staff writer Thomas Fitzgerald at 215-854-2718 or tfitzgerald@phillynews.com.