It was a spring-like Saturday in February. Alexandria Democrats swarmed toward Hammond Junior High School for their first modern mass meeting, instead of the customary primary election, to nominate candidates for mayor and city council in next Tuesday's election. When heads were counted, 1,178 voters had showed up--the largest Democratic mass meeting turnout since 1972.

"Where are all these people coming from?" exclaimed a precinct leader. Was it the weather? It helped. Was it the partisans of the 10 candidates who were vying for seven nominations? They helped, too.

But party leaders sensed that something else was going on: a protest against the Reagan administration.

The force of the protest was felt first in the Tweedledee-Tweedledum race for governor three months earlier, when Democrat Charles S. Robb carried the state and every voting place in Alexandria--even George Mason precinct in Beverley Hills, the city's most Republican neighborhood.

Republican David Speck was a casualty of November's election. A generally respected freshman state delegate, he was beaten by a highly qualified Democratic community leader, Marian Van Landingham--and by the anti-Republican tide. Speck concluded that Alexandria voters were sending a signal to Washington and had used him for the smoke.

Alexandria votes Democratic in presidential and congressional elections. It used to vote solidly Democratic in state and city races as well. But moderate Republican Linwood Holton carried Alexandria and the state in the 1969 contest for governor. And one to three mainstream Republicans have been elected to the city council in each of seven general and special elections beginning in 1967.

Tuesday's results no doubt will reflect the central tendency of the city's whole electorate, rather than the choice of any single territorial or ideological minority.

Voters in these triennial local general elections have in mind real estate taxes, city services, neighborhood protection and political personalities. To an important extent the outcome also reflects swings in the national mood.

Mayor Charles E. Beatley Jr., who seeks a fifth term, stresses this Alexandria-as-a-barometer theme. "I think they'll notice our election results across the river in the White House," he says.

Beatley also points out that the Democratic candidates as a group have a broader and longer record of community service and are philosophically more homogeneous than their Republican opponents. The Democratic contenders, moreover, have never worked so cooperatively at the nuts and bolts of campaign organization.

Like any group whose votes can be identified, blacks hold the balance of power in a close election. Twenty-two percent of Alexandria's 103,000 population is black. They generally support Democrats but tend to "plunk" (cast a single vote) for black candidates.

But Lionel Hope, the only black contender, has been sturdily loyal to fellow Democrats, regardless of race, in previous campaigns. He and his supporters can take heart from the fact that he led the field among nine candidates for council at the February nominating meeting. So blacks can vote for the whole Democratic ticket without fear that they might help elect a white candidate at Hope's expense.

Democrat Patsy Ticer is running a strong race, gathering support from liberals, moderates and conservatives.

Success for the Democratic slate depends on its ability to work together to bring out each member's vote for the entire ticket, and on the desire of Alexandria voters to reinforce the message they sent to the Reagan administration last November.

My prediction? Beatley will win the mayor's race by a small margin. Incumbent Democratic Councilmen Jim Moran and Don Casey will be re-elected. Democrats Hope and Ticer have the best chance among all 10 non- incumbents to pick up the two council seats being vacated.

Council incumbents are hard to dislodge, but Democrats Mark Pestronk and Rich Leibach have the community service and intellectual credentials to beat Republican council members Marlee Inman and Carlyle Ring if the voters want to send a message to the White House.

Beatley is fond of telling a story that is on point: a deep-sea diver is groping along the ocean floor when he gets an urgent message from his tender on deck: "Get back up here in a hurry. The ship is sinking."

The forecast for Tuesday is sunny and pleasant--good weather for sending messages.