Four decades after the first forecasts that Japan's population decline was inevitable, Prime Minister Shinzo Abe is drawing a demographic red line with a target to halt the slide at 100 million people by 2060, a 20 percent drop from current levels.

But holding the line will be difficult and costly unless Japan opens the door wider to long-term immigration, a step policymakers say they have no intention of taking. A proposal floated among Abe's advisers to increase the number of immigrants to 200,000 a year by 2050 was rejected.

"What the prime minister is thinking now is to let foreign workers in for a limited period because of the shortage of construction workers in the run-up to the (Tokyo) Olympics in 2020," said Seiko Noda, head of the ruling Liberal Democratic Party's General Council.