In this glum, depressed Birmingham suburb of silent smokestacks and the unemployed, the black steelworker heaped his plate full of fatback, butter beans and cornbread at what the union hall billed as a "poor man's supper."

For unemployed welder David Rogers, 51, it was a free meal and a chance to rail at President Reagan and his local disciple, Rep. Albert Lee Smith Jr. (R-Ala.).

"I call it 'Smithonomics,' " sniffed Rogers, laid off last summer as a $10-an-hour welder for U.S. Steel after 30 years. "The first thing Smith did when he got to Washington was to vote against extending our unemployment benefits. I'm down to basics now. Every penny I get from unemployment goes for bare essentials. A free meal helps."

Rogers is just one among thousands of laid-off steelworkers here. Many are blacks, whose frustrations pose a threat to the one-term Republican incumbent.

Black voters such as Rogers comprise 28 percent of the population in the economically depressed 6th Congressional District, which is about the same proportion of blacks statewide. This makes them a potential swing vote in this race because the polls indicate that the campaign for the Nov. 2 election is too close to call.

Smith's Democratic challenger, Jefferson County Commissioner Ben Erdreich, 43, is fighting to squeeze enough political angst from unemployed blacks, blue-collar workers and labor to turn the race into a referendum on Reaganomics.

"Are you working? " Erdreich shouts in union halls. This evokes a loud, "No!"

Smith, 51, is asking, as are Reagan and Republican candidates this year, that voters give Reaganomics a chance.

" . . . If we hold our course, will the people of Birmingham, Ala., have the patience to stay with the ship? " Smith's campaign manager, R.T. Gregg, asked.

Smith won his seat in 1980 with 51 percent of the vote against a weak Democrat after unseating moderate Republican John H. Buchanan in the primary election with a big boost by the Moral Majority, which helped turn out the vote.

Smith still battles the problem of being considered a political fluke, but he enjoys the support of wealthy suburbanites, conservative businessmen, a $300,000 war chest twice the size of that projected for Erdreich, and good grades for constituent services.

The key to the black vote is Birmingham's black mayor, Richard Arrington, elected in 1979 after he forged a coalition of whites and 72 percent of the city's black voters. He and others peg black turnout to interest generated by the Alabama governor's race.

Smith strategists are rooting for George C. Wallace to win Tuesday's Democratic runoff election against Lt. Gov. George McMillan. They reason that blacks will stay home on Nov. 2 if forced to choose between the former segregationist governor and Montgomery's Republican mayor, Emory Folmar.

Democrats, on the other hand, argue that black voters will turn out to vote against Folmar no matter who wins the hotly contested runoff.