WASHINGTON — Over the past year and a half, paid employees of ACORN, a liberal-leaning community organizing group, have helped 1.3 million mostly young, mostly poor people register to vote, enrolling more new voters overall than any nonpartisan group.

Why some applications reportedly were signed by Mickey Mouse and supposed members of the Dallas Cowboys, among others, emerged as the latest campaign controversy yesterday when John McCain and Barack Obama sparred over whether the Association of Community Organizations for Reform Now has tried to pad election rolls with thousands of suspect voters.

The fracas escalated as the candidates crammed for the third and final presidential debate tonight.

McCain aides first accused ACORN of misdeeds last week. McCain upped the ante yesterday when he called for an investigation of what he described as "voter fraud going on" in battleground states. He also sought to tie the alleged irregularities directly to Obama.

McCain told a TV station in Orlando, Fla., that the Illinois senator "has had relations with ACORN in the past," and he compared those ties to Obama's previous associations with William Ayers, a Vietnam-era radical who now is a professor of education in Chicago.

But election-law experts say there is a big difference between submitting bad registration cards and casting a fraudulent vote. Thanks to new rules for checking newly registered voters, it is unlikely that bad names will be added to the voter rolls or lead to fraudulent votes, they say.

"Mickey Mouse may show up on a registration list, but he's not likely to vote," said law professor Daniel P. Tokaji of Ohio State University.

Obama, responding to McCain, said he represented ACORN in a lawsuit against the state of Illinois in the mid-1990s to force the state to implement a federal law so people could register to vote when they obtained a driver's license. The U.S. Justice Department was on the same side as ACORN.

"That was my relationship, and that is my relationship to ACORN," Obama said at a resort near Toledo, Ohio, where he is preparing for the debate.

ACORN, Obama added, is not advising him and does not work for his campaign, which has run its own voter registration drive.

Warning that Republicans in the past have employed "voter suppression tactics," Obama added, "Let's make sure everybody is voting, everybody is registered, everybody is doing this in a lawful way."

Obama's aides acknowledged in a conference call yesterday that the campaign paid more than $800,000 to a group affiliated with ACORN to augment get-out-the-vote operations during the Democratic primaries in Ohio, Pennsylvania, Indiana and Texas. The group did not register voters, however.

Mike Slater, head of Project Vote, which helped ACORN run its current registration drive, said the group has identified about 5,000 "bogus or potentially fraudulent" applications so far. In most cases, he said, canvassers copied names from phone books.

Another 65,000 applications have been disqualified because the information on the cards was incomplete, and another 25,000 have been deemed invalid because the voter was already registered, he said.

The group is barred by law from destroying such applications but flags them and notifies local election officials in every case, he said.

Kevin Whelan, a spokesman for ACORN, didn't dispute that the group has submitted improper or duplicate voter registrations this year, but said it fires any canvassers found to be submitting fake registrations and notifies election officials.

Six years ago, Congress passed the Help America Vote Act to make sure states did more to clean up voter rolls. One provision requires states to have a computerized database of its voters. A second says a new voter should not be added to the rolls until the information on the registration card is checked with the state driver's license on file or with a federal Social Security number.

Before computerized databases linked to state records, it was easier to add suspect names or a duplicate registration to a county's voter roll.

"There was a time when you probably could get on the rolls as Mickey Mouse. But the checking procedures are a lot better now because of (the Help America Vote Act). They have improved with each election cycle," said R. Doug Lewis, executive director of the National Association of Election Officials in Houston.