"Improperly calibrated machines do not default in a manner that causes Republican votes to be recorded as votes for other candidates, Democratic or Libertarian," he wrote.

The software is programmed to highlight only those candidates whose names the voter actually touched. The machine may not accurately reflect the voter's intent if it has lost its calibration, Bartlett said. But as a safeguard every DRE voting system used in the state prompts the voter to verify their vote, he said.

Election watchdogs have for some time expressed concerns about DRE systems. Many academics, researches and election watchdog groups such as Verified Voting and Common Cause have said that such systems are prone to errors of the sort claimed by the RNC.

They have claimed that such machines are susceptible to tampering and accidental failure.

Many of the concerns are directed at DRE systems that lack so-called voter verifiable paper audit trails (VVPAT).