With the presidential race in full swing, Colorado and other states have found critical flaws in the accuracy and security of their electronic voting machines, forcing officials to scramble to return to the paper ballots they abandoned after the Florida debacle of 2000.

In December alone, top election officials in Ohio and Colorado declared that widely used voting equipment is unfit for elections.

"Every system that is out there, one state or another has found that they are no good," said John Gideon of the advocacy group Voters Unite. "Everybody is starting to look at this now and starting to realize that there is something wrong."

The swing states of California, Ohio and Florida have found that security on touch-screen voting machines is inadequate. Testers have been able to disable the systems and even change vote totals.

Florida's "hanging chads" in the disputed 2000 Al Gore-George W. Bush election exposed the imperfection of paper ballot counting and helped lead to a $3 billion government initiative to bring voting into the digital age. The Help America Vote Act of 2002 effectively required that states have electronic equipment in place by 2010.

There are no documented cases of actual election tampering involving electronic voting machines.

But in tests, researchers in Ohio and Colorado found that electronic voting systems could be corrupted with magnets or with Treos and other similar handheld devices.

In Colorado, two kinds of Sequoia Voting Systems electronic voting machines used in Denver and three other counties were decertified because of security weaknesses, including a lack of password protection. Equipment made by Election Systems and Software had programming errors. And optical scanning machines, made by Hart InterCivic, had an error rate of one out of every 100 votes during tests by the state.

"I was surprised," Colorado Secretary of State Mike Coffman said Friday of the failures his office found. "It's an awful position to be put in, but I feel strongly it's important that this equipment be secure and accurately count a vote."

Now some states are turning back to paper — in some cases, just weeks before primary elections.

California, Ohio and Florida have chosen to use scanning machines that count paper ballots electronically.

In Colorado, which has spent $41 million in federal grants on electronic systems, many of the state's nearly 3 million registered voters — and the county officials who conduct the voting — don't know what their elections will look like in 2008.

Coffman and Colorado's clerks and recorders are in a dispute over whether to use mail-in ballots or cast paper ballots at polling places.

All fear time is running out.

"We look at each other and go, 'We have used this equipment in three elections. Why did it get taken to a test board and get decertified?'" said Debbie Green, who heads the Colorado County Clerks Association and is the clerk and recorder of rural Park County. "There are some counties having elections in January and February and they don't have any election equipment."

Vendors of the electronic voting machines warn against a rush back to paper.

It can take two years to put a voting system in place, and overhauling a system just weeks before some states hold presidential primaries will invite a new round of problems, they say.

"To throw the baby out with the bath water is certainly shortsighted," said David Beirne, executive director of the Election Technology Council, which represents manufacturers of 90 percent of electronic systems used in the country.

States have their own certification standards, complicating things for manufacturers. "From an industry standpoint, trying to design a voting system when you don't know how it's being judged is causing a lot of problems," Beirne said.

And having a paper ballot does not guarantee security.

"If you look at the history of election fraud, you are really talking about paper," said Merle King, executive director of the Center for Election Systems at Kennesaw State University in Georgia.