A Democratic Party official chided President Reagan's campaign chairman yesterday for sending out a letter asking fundamentalist ministers across the nation to aid in a GOP voter registration drive.

The letter by Sen. Paul Laxalt (R-Nev.) was sent to some 45,000 ministers in 16 states asking them "to play a significant role in what may very well be the most pivotal election of this century."

The July 9 letter was addressed "Dear Christian Leader" and said Reagan "has made an unwavering commitment to the traditional values which I know you share."

Laxalt said he was "challenging you now to organize a voter registration drive in your church."

Gene Russell, communications director for the Democratic National Committee, said the letter "takes a step in mixing religion and politics."

He described the letter as "an unhealthy road for the Republicans to travel."

"The language in that specific letter appears to move beyond the bounds of political discussion," Russell said.

Laxalt's letter said, "As leaders under God's authority, we cannot afford to resign ourselves to idle neutrality in an election that will confirm or silence the president who has worked so diligently on your behalf and on behalf of all Americans."