It is at least partly a reflection of responsible fiscal, monetary, trade, and regulatory policies for which both political parties can rightly claim part of the credit. Moving the federal budget from huge ongoing deficits to substantial surplus has put downward pressure on interest rates, fostered investment, and made it easier for the Federal Reserve to keep the economy from overheating. Serious deficit reduction was kicked off by the agreement between President Bush and the Democratic Congress in 1990 and dramatically pushed ahead by the first Clinton budget in 1993, for which no Republicans voted. The budget agreement of 1997, produced after protracted wrangling between the Republican Congress and President Clinton, completed the job of shoving the budget into surplus for the first time in three decades. Look for all candidates to crow a bit over the surplus and emphasize their own or their party's role in achieving it.
Monetary policy deserves some kudos, too.
The Federal Reserve's interest rate increases in 1994 kept the economy from overheating and arguably allowed growth to keep going until the stars that brought the productivity surge were aligned. The rate reductions of late 1998 stabilized world markets and helped avoid another round of international financial chaos that could have destabilized the United States along with our trading partners. Most candidates will not mention the Fed's role, however, because they cannot claim credit for it and cannot influence it directly, even if they win. Moreover, the president, with the enthusiastic support of the Senate, wisely forestalled any attempts to make the leadership of the Fed an election issue by reappointing Alan Greenspan to another four-year term as chairman. Lowering trade barriers helped, too.