A preelection registration drive by Maryland Republicans is adding thousands of new voters to the party, even in such liberal strongholds as Montgomery County, where for the first time in more than a decade new GOP voters now outnumber new Democrats.

John C. Eisele, a Montgomery elections administrator, said yesterday a preliminary report on voter registration in August showed that 2,638 county residents registered as Republicans while 2,564 registered as Democrats.

The Montgomery totals marked the first time in about 15 years that new Republican registrants outnumbered their Democratic counterparts in a single month, Eisele said.

Some local Republican leaders hailed the report as a sign of the party's good health in the overwhelmingly Democratic county. Others went so far as to say the large number of new registrants guarantees that President Reagan will take Montgomery in November, as he did in 1980.

"You can bet the ranch on it," declared state Sen. Howard A. Denis, one of two Montgomery Republicans in the General Assembly. "The new numbers are a sure sign of a landslide of epic proportions. It's going to change the way a lot of people think about Montgomery County."

While Democrats still make up 57 percent of Montgomery's electorate, there are now 102,939 Republicans in the county, nearly 9,000 more than the 94,101 of four years ago. In 1980, when Republicans accounted for 29 percent of the voters in Montgomery, Reagan won Montgomery with 48 percent of the vote.

John T. Willis, the Maryland campaign coordinator for Democratic presidential nominee Walter F. Mondale, said the Republican increase in Montgomery "is not that big a concern."

In contrast to Republicans, whose registration drive included canvass days over several summer weekends, Democrats, Willis said, "were concentrating on our intra-party fights."

Mondale campaign officials will soon launch a five-week voter registration drive of their own, Willis said.

State and local elections officials said the Republicans' August increase in Montgomery reflected a similar pattern around the state, as a proportionately higher number of voters are registering as Republicans.

"The number of Republicans is definitely increasing, but the Democrats still have the hefty numerical advantage," said Carol S. Evans, deputy state elections administrator.

In July, 6,663 Maryland residents registered as Democrats, while 3,896 registered as Republicans, said Evans. Statewide that month, there were 1.5 million Democrats and 488,684 Republicans.

In heavily Democratic P.G. County, 1,526 people signed up in August to vote as Republicans, 3,897 as Democrats, said elections administrator Robert J. Antonetti.

However, despite that nearly 3 to 1 margin in favor of the Democrats, "the Republicans for the first time have really mobilized their registration effort," Antonetti said.

Bo Denysyk, who is managing Reagan's reelection effort in Maryland, said Montgomery residents who were targeted in the recent registration drive included "the independent voter, new arrivals, first-time home buyers, people in Rockville" and employes of the county's numerous defense contractors.

"In the past three months, we've been breaking even or slightly ahead of the Democrats" in new registrants in Howard, Harford, St. Mary's and Anne Arundel counties, Denysyk said.

Come November, "I think we're going to take the state," Denysyk added.