After nearly 40 years of pressing the levers on mechanical voting machines that once were hailed as the wave of the future, this year Montgomery County voters will begin converting to the latest thing in elections - the computer-based voting system.

The phasing-in of the new method will be confined this year only to the 17th election district - centered in Rockville - where residents will punch computer cards that will be transported to the county's data processing center for tabulation.

Although the new procedure was adopted by election officials for its significantly lower costs and its ability to store vote tallies, the latest style will, as such modes often do, transform the traditional election day spirit that emanates from the polling place.

Probably the first tradition to disappear will be the one-to 21/2-hour wait - both a source of agitation for some hurried voters and neighborly chatter for others who engaged in spontaneous coffee klatches while waiting in line.

The new, cheaper punchcard machines will "virtually wipe out the lines" said Election Board Chairman Marie Garber, because more machines can be purchased. Each one of the lever-type systems cost about $2,300, while each one of the new machines, encased in a suitcase, is priced at only $180.

In addition, now after the polls close, machines are opened and quickly tallied, sending nervous poll watchers rushing to their telephones to party headquarters. This precinct mystique will vanish, too, because the votes will be counted by computer in the sequestered data processing center in Rockville.

"This will be a big difference from the tradition in this county," acknowledged Garber to the County Council.

"Yes, it will," murmured council President Elizabeth Scull, nodding her head and fingering the punching device that will mark the new ballots.

The electronic system works this way: A voter will be given one or more computer cards marked with candidates' names and ballot questions. Instead of entering a booth hidden by a curtain, the voter will approach a small open suitcase, with raised sides for privacy, and indicate his choices by punching holes in the cards.

The voter will deposit the ballots in a box, which will be carried to the computer counting center for tabulation. An important attraction of the new method is the computer's ability to store the vote results permanently, if necessary.

"The machines we now use leave no audit trail," Garber explained. "If a machine malfunctions, the votes are lost and we don't even know it. If an election is challenged, there is no way to recount it . . . With the new system we have documentation of the full vote of each voter."

The Datavote system-selected by the county election board is a "proven" method which has been in use in several Western and Sun Belt states for years, she said. It is one that can be modified in height to accommodate the handicapped and, at 22 pounds per suitcase, can be transported simply.

"And what about security and fraud?" council member Dickran Hovsepian asked Garber, eyeing the portable voting instrument. "We lock the voting machine once everything is set up. Do you lock the computer?"

Responded Garber emphatically: "There are many checks and protections in the system to ensure there will be no corruption."