An unprecedented last-minute surge of voter registration in Maryland, Virginia and the District of Columbia has swelled voters' rolls in the area to record levels for the 1980 presidential election, apparently reflecting a national trend.

The largest growth took place in traditionally Repubican areas -- Fairfax County and the Baltimore suburbs -- prompting optimistic pronouncements from supporters of Ronald Reagan, but also giving rise to warnings that the registration figures in 1980 may be harder than ever to interpret. Election officials noted that undecided voters are prevalent in both major parties, and cross-over voting in the presidential race could cancel out apparent growth in party strength.

In addition, because of flagging enthusiasm for the major candidates, local election officials predict low voter turnout in November. With the record high regiatration levels, that could mean an all-time low turnout rate in the area.

Democratic Party officials in Maryland, attempting to put the best face on the registration figures, maintain that many of the gains are theirs even though they showed up in traditionally Republican areas. The Democrats and the National Association for the Advancement of Colored People targeted Maryland in recent registration drives.

Nevertheless, Republicans in Virginia and Maryland were buoyant on hearing the preliminary reports of registration figures.

"I think these figures are signifcant and I think they show a swing toward Ronald Reagan in the state of Maryland," said Republican Party chairman Allen Levey. In the first seven months of 1980, Republicans outstripped Democrats in voter registration for the first time since the Eisenhower era, but Democrats regained the lead in August, state figures show.

University of Virginia political scientist and pollster Larry Sabato called that state's registration growth a positive sign for Reagan and other Republicans "since the suburbs have provided the margin of victory for every Republican candidate since 1969."

"It's a little scary," a national Democratic Party official said when he heard about the Maryland figures. "We're fearful, of course, that a lot of these registrants are Republicans."

Official figures will not be available until next week, but preliminary totals indicate voter registration in Maryland will break the 2 million mark for the first time, state election law administrator Willard Morris said. That represents an increase of at least 150,000 voters since the 1976 election, which Jimmy Carter won by 87,000 votes.

Virginia officials said registration since 1976 had grown by 30,000 in late September, not counting the large surge in the days before the rolls were closed last Saturday. The largest increase came in Fairfax County, by far the state's biggest jurisdiction, where 11,800 people registered on the last day. Total registration in Fairfax is at least 27,000 more than in 1976, officials said. Registration also sharply increased in Prince William and Alexandira, officials said.

In D.C., more than 38,000 voters registered in the month since the Sept. 10 local elections, according to election administrator Mary Rogers, who said she had never before seen such a large turnout in the closing days of registration, even in other presidential election years. There, as in Maryland, the rolls were closed Monday. The large registration in the area is apparently part of a national pattern, according to Tracy Gallagher, director of voter registration for the Democratic National Committee, citing similar late surges in New York, Texas, Ohio and Cleveland. Those states, like Maryland, were targeted by the Democrats in a national registration drive aimed at the presidential election.

Some officials found the high registration levels perplexing because so many voters remain undecided, and apparently unenthusiastic about the presidential race, with less than a month to go until election day.

"People were coming in all day Monday and they just volunteered to our election officials that they don't know whom they're going to vote for, but they certainly do plan to vote," said Betty Eby, election adminstrator in Maryland's Anne Arundel County.

That traditionally conservative county, which went for Gerald Ford in 1976, registered 6,280 voters on closing day Monday. Eby predicted that final tallies will show an increase of 7,000 to 10,000 voters since the 1976 election.

The increases in Anne Arundel and in Prince William, one of Northern Virginia's fastest-growing outer suburbs where registration is up by 9,000, were attributed partly to population growth.

In Maryland, purges of voter rolls and the convenient registration-by-mail process also spurred the registration surge.

Maryland counties regularly purge their rolls of all voters who haven't voted in the last five years. Many voters who registered this year are probably longtime residents who have sat out recent elections and had to reregister in order to take part in the 1980 election.

Those purges wiped more than 16,000 voters from the rolls in the Democratic stronghold of Baltimore City -- the state's largest jurisdiction -- in the first six months of 1980, and also canceled out much of the increase in registration in Prince George's County, officials said.

More than 4,000 voters registered in Baltimore on Monday, capping what officials there called a record surge in registration in the previous 10 days. The Democratic Party said it registered 12,000 Baltimoreans and 16,003 statewide, and that NAACP claimed to have signed up 10,000 more as part of its national registration drives. Organized labor added another 3,727 statewide.

Nevertheless, city election officials said registration will probably total no more than 410,000 -- a loss of at least 8,000 voters.

Meanwhile, in Baltimore County, where Ford defeated Carter in 1976, preliminary figures show an increase of 24,000 voters. Officials cautioned that the country has not conducted a five-year purge this year, so that figure may be somewhat inflated.

In traditionally Democratic Montgomery County, officials predict a 13,000 increase in registered voters, but Republicans account for a larger percentage of the new voters than they have in past years.

"In the past, this sort of pattern has not bode well for the Democrats," said Montgomery election administrator Marie Garber. "There was a similar pattern in 1972, when the county went for Nixon. Democrats had a high percentage of the registration in 1976, and the county went for Carter."

Virginia officials predicted that the registration pattern there could spell bad news not only for Carter but also for Congressman Herbert Harris, the 8th District Democratic incumbent.

The southern Fairfax portion of his district, a conservative Republican stronghold, was one of the main sites of increased registration.