In the most recent public poll, released Thursday by Franklin and Marshall, President Obama is ahead of Romney, 44 percent to 38 percent, with 15 percent undecided. The poll's margin of error is plus or minus 3.8 percentage points.
A Republican Party spokeswoman said the GOP was challenging the nominating papers because they are riddled with errors, and the party is concerned that Democrats are behind the petitions.
The petitions, spokeswoman Valerie Caras said, were circulated by and signed largely by Democrats.
The president of the Libertarian Party of Pennsylvania, Tom Stevens, said the petition drive was financed strictly by Libertarian Party members and the campaign of former New Mexico Gov. Gary Johnson, the party's candidate for president.
"It's part of no plot, and no money is coming from Democrats to finance our petition drive," Stevens said.
He said it may be true that the signers and circulators were Democrats. But they were hired exclusively by a contractor for the national Libertarian Party, and they must accurately represent the Libertarian Party and its philosophies, he said.
Johnson ran for president as a Republican last year before dropping out ahead of the primaries. The Constitution Party candidate for president, Virgil Goode, is a former Republican congressman from Virginia.
Neither man is considered the kind of threat posed by Ross Perot, whose independent campaign attracted nearly 19 percent of the national vote in 1992, the year Republican President George H.W. Bush lost to his Democratic challenger, Bill Clinton.
The Republican Party's petition challenge also includes the Libertarian Party candidate for U.S. Senate, Rayburn Douglas Smith, and for attorney general, Marakay Rogers. It also challenges both Libertarian and Constitution candidates for state treasurer and auditor general.