Preaching to the converted is every local politician's stock in trade, a source of campaign contributions and workers. Montgomery County Del. Lucille Maurer, who expects soon to be in the fight of her political life, preached with gusto earlier this week, raising more than $10,000 in the space of two hours.

Surrounded at a cocktail party in Silver Spring by 200 of her closest Democratic friends and a couple of token Republicans, Maurer described her 25 years in office -- on the county Board of Education and as a senior member of the Maryland House of Delegates -- as "a rollercoaster."

Then, sweeping her arm toward a crowd that included a congressman, the state's lieutenant governor and her legislative boss, House Speaker Benjamin L. Cardin of Baltimore, she added with a grin: "This is surely a high point."

If Maurer were running again for delegate, the money raised at that one celebration would have virtually guaranteed her re-election.

Instead, she wants to become the next state senator for District 19 in eastern Montgomery, replacing incumbent Sidney Kramer, who is running for county executive. For that, Maurer reckons she needs about $50,000.

The race, which already is drawing attention from political groups across Maryland, may be one of the most expensive Senate campaigns in Montgomery history because it pits Maurer, 62, against Del. Idamae Garrott, 68, a fellow Democrat who enjoys enormous popularity among voters in the generally middle-class district.

Both women are liberal Democrats with strong ties to such special interest groups as county teachers and civic associations. But they are quite different in their approach to politics, representing opposite ends of a local party that has been split since the late 1960s, when it was controlled by Democrat Richard Schifter, with whom Maurer was allied.

The contest between Maurer and Garrott holds such promise as a grudge match -- a rare last hurrah for two political veterans -- that even those Montgomery Democrats who are busy plotting their own re-elections in 1986 are taking notice.

"It's going to be hot," said Del. Michael R. Gordon, a Rockville Democrat who attended Maurer's fund-raiser. "My parents live in this district; they're my barometer. They're so used to voting for both Lucy and Idamae. Now they can't make up their minds."

Indeed, some observers say the most refreshing thing about the forthcoming campaign is that for the first time in a great while, two articulate Democrats will be forced to sell themselves and their ideas to voters other than party regulars.

"Both are going to have a good share of party folks," said Montgomery Democratic Chairman Jay S. Bernstein. The winner, he added, will be she who successfully "reaches out beyond the party."

That is a tall order for a district as unpredictable as the 19th, where Maurer and Garrott have always split the top vote. In 1978, the year Garrott was first elected to the House, she had 5,942 votes, 547 fewer than Maurer.

Four years later, though, Garrott, a tireless campaigner with a knack for siding on the popular end of an issue, was re-elected with 9,662 votes, 869 more than Maurer.

Adding uncertainty to the race is what some perceive to be a growing conservative element in the district, where Republican registration jumped from 13,000 in 1978 to more than 17,500 today. (Democrats still outnumber Republicans in the 19th by a roughly 2-to-1 margin but have added only 1,500 new members in the past six years.)

There were few new faces at Maurer's fund-raiser this week, just the "heart and soul" of the powerful Democratic Party, stalwarts such as county planning board chairman Norman L. Christeller and state Democratic Chairman Howard J. Thomas.

Maurer thoroughly enjoyed the evening, surrounded by her political friends. After all, both she and Garrott have more than a year to appreciate the changing face of their electoral district.