Slight, bespectacled Sandra McHugh doesn't look frightening, but as the November election approaches in Maryland, it's people like her who haunt Democratic incumbents' dreams.

"I grew up in Baltimore, and I was a Democrat, but that was mostly because my parents were Democrats," said McHugh, 31, who has since moved to booming, suburban Howard County, registered Republican and voted for Ronald Reagan and George Bush. "My views changed: I like the party's positions supporting business."

Voters such as McHugh and her husband inspire the giddiest hopes of Republicans in this longtime Democratic stronghold. Young professional couples moving into fast-growing outer counties have driven Republican registration in Maryland steadily upward for the past decade, eroding the statewide Democratic edge from 3 to 1 in 1980 to 2.19 to 1 in 1990.

It's a trend that has been strongest where growth has been most vigorous, places like Howard County, where the Democratic advantage has been almost halved in the last decade, and Carroll County -- northwest of Baltimore -- where Republicans now hold a 700-voter edge. In Montgomery County, 69 percent of the last decade's new registrants have been Republicans.

So far, the GOP converted few of these triumphs at the registration tables into victories at the voting booths, but now -- on the eve of an election with a group of GOP challengers that is larger and stronger than it has been in years -- Maryland Democrats are worried.

"I knock on doors, and they often say, 'Oh, wrong party,' and you have to talk fast -- tell them about your record, what you stand for -- before they close the door on you," said state Sen. Edward J. Kasemeyer (D-Howard), who is being hard pressed by Republican Christopher J. McCabe. "That didn't happen before."

The conventional wisdom has been that party affiliation has little meaning in local Maryland politics, but people watching Democrats' numerical strength weaken wonder whether those days are ending.

"We're not going to be a two-party state tomorrow, but the Republicans are creeping into the take-off stage," said Herbert C. Smith, a political scientist at Western Maryland College.

For example, in Anne Arundel County, polls show Republican Robert R. Neall in a virtual dead heat with Democrat Theodore J. Sophocleus for county executive, an office the GOP lost eight years ago. And in Baltimore County, Republican Roger B. Haden is mounting a strong challenge to County Executive Dennis F. Rasmussen (D).

The Baltimore County contest "is a close race, an awful lot closer than any of us would have thought," said state House Majority Leader John S. Arnick, a Baltimore County Democrat. "Out here it's revolt time. For the first time since Spiro Agnew was county executive here, we're looking at the possibility of a Republican in that job."

"The last couple years has finally waked up the Democratic Party that we'd let things slip," he added.

In Montgomery County, meanwhile, Nancy Dacek and Betty Krahnke have good chances of becoming the first Republicans on the County Council in 20 years. For the first time ever, the county is electing members by district, rather than countywide, a change that helps the GOP.

"In certain areas like Germantown and Gaithersburg, we've stopped doing voter registration because all we end up doing is registering Republicans," said Gail Ewing, a Democrat running for an at-large council seat.

Republican leaders like to point to races such as the Kasemeyer-McCabe state Senate face-off in Howard. They also have high hopes in Howard for Martin G. Madden, who is challenging Democratic Dels. Robert J. DiPietro and William C. Bevan in District 13-B.

The GOP brass are putting most of their emphasis on creating "a farm team." State party Chairman Joyce L. Terhes explained, "We're going to build from the bottom up, from the courthouses on up. This, to me, is the foundation year, then we gear up for 1992 and the presidential election. Then 1994 could be a great year."

Republicans already point to victories in places such as Frederick County, where two Republicans sit on the five-member Board of Commissioners and another is mayor of the city of Frederick, and in Calvert County, which has two Republicans on a five-member commission.

But for now, Maryland remains largely a one-party state. Democrats have an 88 percent majority in the General Assembly. Both the state's U.S. senators are Democrats and so are six of its eight members of the U.S. House. Democrats also control 83 percent of county offices, according to Nathan Landow, chairman of the Maryland Democratic Party.

Maryland's Democratic traditions stem in part from its urban, industrial core. Although the Washington and Baltimore suburbs are gaining more clout, Baltimore remains the political center of the state. Over the years, the city's strongest voting blocs have been labor union members, white ethnics and blacks -- groups that tend to vote Democratic in local elections.

Nevertheless, Landow said he is concerned about the GOP gains and plans to target younger voters. "Many of them are not as concerned with social issues as with the economy. And Republicans have had better control of that issue," he said. "But with the budget situation and the economy now, that's beginning to change."

Sen. Paul S. Sarbanes (D-Md.) seems less worried. "The people who used to register as Democrats so they could influence the primaries and then vote Republican in the general election are just registering Republican now," he said. "As long as we keep running good candidates on the local level, they'll vote for us."

There may be good reasons for lingering Democratic complacency. How worried should Democrats be about a GOP state organization that announced last spring that it was out of debt for the first time since 1986 and had just bought its first fax machine, a party still so hungry for candidates that it resorted in Prince George's County to using newspaper ads to solicit them?

Political scientists puzzle over why the Republican Party hasn't done better in Maryland.

"Nobody's explained it. The demographic profile of the state, in terms of education and income, should make this a two-party state. It's not that different from Pennsylvania to the north and Virginia to the south," Professor Smith said.

Some within the party attribute its troubles until recently to factional fighting between moderates and extremists and to the practice of backing prominent outsiders for high office.

Senate nominees Linda "Chavez and {Alan} Keyes were good, but they have no home-grown roots," said David R. Blumberg, GOP chairman in Baltimore.

But whatever its shortcomings, the Republican Party's strength in the growing suburbs is unsettling to Democrats such as Joyce Kelly, president of a Howard County community organization.

The Republicans "are here and they're aggressive. They're the part of the me-first generation, part of the '80s," she said. "They say, 'Let me in {to the suburbs} but stop the growth. Let me get there, but I don't want to deal with money for the schools.' Where is all this leading?"