GOV. L. Douglas Wilder (D) of Virginia, sounding more and more like a presidential candidate, says he will decide by Labor Day whether to challenge President Bush in 1992.Mr. Wilder, the nation's first elected black governor, praises former President Harry Truman as the kind of frugal, yet compassionate, leader the nation needs. At a breakfast meeting with reporters yesterday, Wilder moved both left and right of Mr. Bush. He chided the Bush administration for siding with anti-abortion demonstrators against a federal judge in Wichita, Kan. At he same time, he said the White House has allowed the budget deficit to balloon to $348 billion by failing to cut waste. Wilder is one of a shrinking list of Democrats who are considering a bid against Mr. Bush. This week, Sen. John D. (Jay) Rockefeller IV of West Virginia took himself out of the running. Earlier this summer, Rep. Richard Gephardt of Missouri, the House majority leader, also decided against a race. Sen. Lloyd Bentsen of Texas, who was the Democrats' vice presidential nominee in 1988, seems disinterested in another campaign, even though some members of his party think he would be a solid contender. That leaves just a few names besides Wilder: * Former Sen. Paul Tsongas of Massachusetts. So far, he is the only Democrat officially in the 1992 campaign, and he says he is beginning to feel "lonely" out there on the trail. * Sen. Tom Harkin of Iowa. A fiery populist, Senator Harkin would take the fight directly to the president. But his liberal credentials raise doubts about his strength outside the Midwest. * Gov. Bill Clinton of Arkansas. He would bring a moderate voice to the campaign. He promises to decide whether to run within "a couple of weeks," according to the Washington Post. * Gov. Mario Cuomo of New York. Although he says there are no plans for a campaign, many Democrats say he would be their strongest possible candidate. Because he is well known, and could tap New York's deep financial pockets, Mr. Cuomo could start the race late and still run strongly. * Former Gov. Jerry Brown of California. Mr. Brown is looking at a possible campaign. * The Rev. Jesse Jackson. Washington's "shadow" senator may run, but some Democrats doubt he will campaign this time.