After the botched 2000 presidential election, when confusing ballots in Florida and legal wrangling left the country in political limbo for 36 days, the federal government set aside billions of dollars to buy modern voting machines.
But experts say the computers - which leave no paper record of ballots - have gaping holes in their security that would allow hackers to tamper with the vote count.
Forbes magazine called paperless voting a "worst technology" of 2003 and three bills are before Congress to stop it.
"The machines are in use with what I consider extremely serious security flaws," University of Iowa computer professor Douglas Jones said.
He said he told the machines' manufacturer, Diebold, about the security problems five years ago, but nothing had been done to fix it.
"The machines are in use with what I consider extremely serious security flaws"
computer professor, University of Iowa
The new machines will be used in Tuesday's primaries, when Democrats in Delaware, Missouri, Arizona, Oklahoma and South Carolina will choose who they want to challenge George Bush in the November election. New Mexico and North Dakota hold caucuses the same day.
Another study at Johns Hopkins University uncovered other problems with electronic voting.
Professor Aviel Rubin said the voting machines were equipped with modems and could be instructed to do nearly anything over a telephone line.
A hacker could give a candidate extra votes or disrupt an entire election, in the same way the Mydoom virus had interefered with computers around the world, he said.
Unscrupulous election officials could steal an election, he said. Rubin's team could even vote multiple times - just by forging a "smart card," which each voter inserts in the machine before voting. He said even a teenager could do that.
What really concerns Rubin is that machines leave no paper record of votes, making recounts impossible.
Voter activist Victoria Collier said 80% of the 33,000 electronic machines in use so far were made by just three companies: Diebold, EES and Sequoia. They are heavy Republican donors, she said.