The poorest one-tenth of Americans will pay 20% more of their earnings in federal taxes next year than they did in 1977 and the richest will pay almost 20% less, the Congressional Budget Office said. In an analysis of federal tax burdens, the nonpartisan research agency concluded that a middle-income family (about $30,000 income in 1988) will pay 20.9% of its earnings in federal taxes next year, down slightly from 21% in 1977. Although the tax overhaul enacted last year is expected to make the system more progressive, the report added, ability to pay will still be less of a factor than it was in 1977. The report was requested by Sen. George J. Mitchell (D-Me.), a member of the Senate Finance Committee, who favors shifting a greater share of the tax burden onto upper-income Americans.