Declaring that Prince George's County is "a sleeping giant" of unregistered black political power, the Rev. Jesse L. Jackson yesterday urged residents to get behind the county's voter registration drive that so far has signed up only 5,000 new voters toward its goal of 30,000.

Reminding a Brentwood church group and a Fort Washington rally that "men and women have died for the right to vote," Jackson said that Prince George's residents had only to sign up to be registered.

A few did. In the crowd of 150 attending the services at the First Baptist Church, 4009 Wallace Rd., North Brentwood, about half a dozen persons stepped forward to register. The others said they already were registered but would work to register friends and family.

The results were the same at the Friendly High School, 10000 Allentown Rd., Fort Washington, where a crowd of about 1,500 heard Jackson.

"Most people at the school rally were already registered," said Alvin Thornton, executive director of Operation Big Vote, the coalition of groups seeking to increase voter participation in Prince George's, which has one of the largest groups of unregistered potential voters of any Maryland jurisdiction.

Operation Big Vote, which has the backing of a variety of organizations including the NAACP, the National Education Association and Jackson's Rainbow Coalition, announced in April that it wanted to register 30,000 voters for the fall elections. The drive is especially geared toward black residents, who have not registered or voted in proportion to their population.

But Thornton yesterday acknowledged that registration is running behind schedule.

"Through mid-July we had registered about 5,000 new voters," Thornton said. "We expect to have a total of 10,000 to 15,000 registered by Aug. 11, the deadline for registration for the September primary." That last-minute surge in voter registrations is possible, he said, because Operation Big Vote has completed research identifying unregistered residents.

"Now that we know who they are, we are sending teams out to register them," he said.

Thornton said Operation Big Vote will work to turn out the vote for the primary and to register more voters in time for the general election. "We aren't closing; we plan to continue to improve voter participation," he said.

Despite yesterday's rally results, Jackson proclaimed the response as "tremendous," saying that what was important was the pledge from the people in the audience to get unregistered residents to register and go to the polls.

"We're building district by district and state by state," he said in a brief interview after the church meeting, where he was introduced by Bennie L. Thayer, a Landover businessman who is chairman of the Prince George's and Maryland Rainbow Coalition and a candidate for the Democratic Central Committee-At-Large, 23rd District.

In his 45-minute presentation, Jackson urged the audience to "organize . . . don't criticize." He said that the vote can be used to right social wrongs and to affect foreign and domestic policies.

"We have a rod, and we have to use it," Jackson said.