PHOENIX — Gov. Janet Napolitano vetoed a series of changes designed to tighten drunk driving laws Tuesday because she didn’t like one provision she said moves in the opposite direction.

 The governor said it is “premature’’ for lawmakers to partly rescind a new state law which requires motorists convicted of driving while intoxicated to install an ignition interlock on any vehicle they drive for a full year after their driving privileges are restored. That device prevents a vehicle from starting unless the driver provides a “clean’’ breath sample. HB 2395 would have cut that to six months in certain circumstances.

 Napolitano said she agrees with those who believe the interlock has a deterrent effect, convincing people not to drink and drive. “Some disagree and believe the penalty is too harsh and the benefit to public safety is too low,’’ the governor wrote in her veto message. But she said the law, which it took effect only last fall is too new to tell, which is why she doesn’t want to alter it just yet — if ever.

 The decision provoked a sharp reaction from House Speaker Jim Weiers, R-Phoenix, one of the architects of the measure. He said Napolitano ignored the fact that the six-month option would be available only to those who complete a drug or alcohol education or treatment program approved by the state Department of Health Services.

 That, said Weiers, becomes an incentive for people to change their behavior.

 Sen. Linda Gray, R-Glendale, said Napolitano’s decision to veto the bill undermines some necessary fixes to drunk driving laws.

 For example, the law currently says motorists convicted of a first-time drunk driving offense lose their right to drive for 30 days and then can drive only to and from work or school for the next 60. But that option for limited driving privileges is not available to anyone whose actions have resulted in “serious physical injury’’ to another.

 Gray said that wording left a big loophole: A motorist could kill someone else and still be entitled to partial reinstatement of driving privileges after 30 days.

 Another provision clarified the procedure for summary suspension of a suspected drunken driver’s license 15 days after the incident.

 And HB 2395 also contained extensive changes in state laws dealing with those who operate a watercraft while intoxicated, stiffening the penalties so they fell into line with those convicted of drunken driving.

 Weiers said he does not know if there is enough time remaining this session to recraft the legislation and send it back to Napolitano, this time without the changes in the interlock law she found offensive.