THE United States government is borrowing money like never before. The national debt rose by more than a third over a one-year period, far more than it ever did at any time since World War II.

In the past, when the government became a heavy borrower, there was talk about crowding out private borrowers. But this time, interest rates have remained low and no one seems to be worried about that.

The reason is simple: Rather than crowding out the private sector, Uncle Sam is now standing in for it. Much of the government borrowing went to investments in financial institutions needed to keep them alive. Other hundreds of billions went to a variety of programs aimed at stimulating the private economy, including programs that effectively had the government pick up part of the cost for some home buyers and some auto buyers.

This week, the Federal Reserve published its quarterly report on debt levels in the economy. While Uncle Sam borrowed more, others borrowed less. The accompanying chart shows that total domestic debt  the amounts owed by individuals, governments and businesses  climbed just 3.7 percent from the second quarter of 2008 through the second quarter of this year. That is the smallest increase since the Fed started these calculations in the early 1950s.