The preliminary unemployment rate for March is the lowest since 1976, when current record-keeping began. Glenn Mills, chief economist with the Maine Department of Labor, said the rate of 2.7 percent could be the lowest since 1957, when President Dwight Eisenhower was in the White House. Mills didn’t have monthly estimates for the years before 1976, but based on annual averages and other records of the 1950s, ’60s and early ’70s, he arrived at an educated guess of 1957. The state’s monthly unemployment rate continues a streak of 28 consecutive months where rates have been below 4 percent.