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Ronald Reagan and Barack Obama have at least one similarity. They both were confronted by great economic challenges when they became president.

Mr. Reagan's immediate challenge was that inflation and interest rates were out of control. He met this great test by allying with the Federal Reserve chairman, Paul Volcker, in accomplishing a return to price stability, even through the 1982 recession when the unemployment rate hit 10.8 percent.

Reagan's success is not in doubt. Inflation and interest rates were reduced dramatically, and the recovery from the end of 1982 to the end of 1988 was strong and long with an average growth rate of real GDP of 4.6% per year. Moreover, Reagan focused on implementing good economic policies, not on blaming his incompetent predecessor for the terrible economy he had inherited.

Mr. Obama was equally in position to get credit for turning around a perilous economic situation that had been left by a weak predecessor. But he has pursued an array of poor economic policies, featuring the grand Keynesian experiment of sharply raising federal spending and the public debt. The results have been terrible and now, two and a half years into his administration, Mr. Obama is still blaming George W. Bush for all the problems.

Friday's downgrade of the U.S. credit rating by Standard & Poor's should have been a wake-up call to the administration. S&P is saying, accurately, that there is no coherent long-term plan in place to deal with the U.S. government's fiscal deficits.

Robert Barro is a professor of economics at Harvard University and a senior fellow of Stanford's Hoover Institution. To continue reading his opinion piece in The Wall Street Journal, click here.