The 1980 census is expected to show that blacks make up 11 percent of Montgomery County's population, making them a potentially powerful political force in the coming decade, county residents attending a voter registration conference last weekend were told.

But if blacks don't register and don't vote, the increase -- up from 4.7 percent of the county population in 1970 -- won't mean very much, warned speakers at the gathering, sponsored by the Montgomery chapter of Alpha Phi Alpha, a black fraternity.

Though the proportion of blacks in the Montgomery population has climbed significantly, blacks hold none of the county's approximately 40 elective offices.

The conference was aimed at "trying to get people geared up for this election year" and to "encourage blacks in the county who may have political ambitions to run," said DeVance Walker Jr., who led the program.

Blacks hold a decisive vote in many of the nation's largest cities and some southern counties, said Malachi Knowles of the Joint Center for Political Studies, a national organization that provides assistance to minority elected officials. But when blacks don't vote, "our potential is diminished," he said.

Knowles urged blacks to "stop shucking and jiving, bucking and backsliding . . . and do something about our political future. We've got to use it or to lose it."

Organizers of the program at Northwood High School in Silver Spring said they feared that a lot of blacks who have moved into Montgomery County recently have not registered.

What the black population increase could mean in political clout is unclear. Some participants said Montgomery blacks generally have good educations and high incomes and will demand more of the elective system.

Others said those same factors could cause apathy among those who have already "made it" personally and are less concerned with political gains.

Knowles, Maryland's Sen. Clarence M. Mitchell III (D-Baltimore) and Rep. Walter E. Fauntroy (D-D.C.), spoke to about 200 persons attending the conference.

Economic achievement has eluded black Americans, Knowles continued. Blacks made gains in civil rights in the 1960s and the movement went "from the streets into the halls of government" in the 1970s, he said. But the challenge of the 1980s is to "parlay our political toehold into greater economic advantage."

'When we fail to get our plans and priorities into the (federal) budget, we fail to get into the ballgame. The only thing left is to play catch-up," Knowles said.

This will be increasingly difficult, he warned, because the 1980s will be a decade of limited growth, fiscal conservatism, energy conservation, rising inflation and greater unemployment.

"Blacks will push for more of the pie, but on the other hand we'll be told the pie is shrinking," he said. "The answer lies with you."

Fauntroy attempted to demonstrate the impact of voting power on the system. He regaled the crowd with an account of how he strongarmed support in Congress for the D.C. Voting Rights Amendment, which so far has found limited support in state legislatures where it must be ratified.

When former D.C. House District Committee Chairman John L. McMillan (D-S.C.) refused to support Fauntroy's voting rights amendment, Fauntroy and other D.C. activists rallied black voters in McMillan's home district, successfully unseating him in 1972. Then, Fauntroy said, he asked black leaders across the country to write their congressional representative urging support for Fauntroy's bill. He asked each to include a reference to McMillan's defeat.

"There are more than 100 members of Congress who can't get elected unless they have black votes," Fauntroy said. "It makes a difference when we register and vote and when we are active. We are sitting on a gold mine in this country but everybody else is digging it out."

Clarence Mitchell described the exchange of favors for votes that makes state-level politics tick. He said he maintains a card file of persons who have made donations to his campaign. When constituents call for assistance with problems, he said, he checks his file first.

"You've got to put your money where your mouth is," he said. "Those are the political realities." The answer to slow economic development is political achievement, he said. "A greater vote in policy-making decisions in this state and this country at large is in your hands."

As if to underline the afternoon's theme, several elected officials and a candidate for office attended the program.

County Executive Charles W. Gilchrist commented, "The black community has been more and more interested in political affairs and it's been all to the good. I think they will be increasingly important politically as well as otherwise."

Mello Cottone, a Silver Spring attorney seeking the Democratic nomination for U.S. Senate in Maryland, worked the crowd seeking support.