The noted American political philosopher Yogi Berra said, "Prediction is difficult, especially about the future."
In the political world now, with just three weeks to go before midterm elections, all anybody is asking about is the latest predictions.
Will Democrats take control of the House? How about the Senate? What issues will matter most to voters? And can President Bush and his allies do anything to affect the outcome?
It is that last question that is at the heart of any analysis (not prediction) about the majority party's prospects for changing a terrible situation into a merely bad one in the last 21 days.
Democrats, everyone agrees, will gain seats, but the question is, how many?
The Democrats' advantages are well known: the president's low approval ratings; the war in Iraq; economic uncertainties over the cost of such life staples as gasoline and health care; a series of ethics controversies involving Republicans; the majority party's long reign in power; and the restiveness on the part of some conservatives disappointed in the administration's record on issues such as deficit spending and immigration.
There are even indications that some strategists are looking at the previous estimates of the maximum number of seats that Democrats can pick up in the House and saying they might be revised upward.
The minority party needs to pick up 15 seats to take control. Up until now, those most bullish on the Democrats' chances saw 30 seats as the most they could win.
Now some are thinking that additional Republican-held seats might be in play. And for the first time all election season, Democrats appear within striking distance of taking Senate control.
Still, there are, to use a seven-dollar word, asymmetries in this election that are likely to benefit Republicans, which is why no one should be making firm predictions yet.
MORE MONEY: Republicans enjoy an overall cash advantage, which is especially large among their House candidates in the most competitive races.
BETTER ORGANIZATION: As they proved in 2002 and 2004, Republicans are simply better at the so-called ground game -- reaching voters in the run-up to Election Day with targeted messages meant to inspire them to vote, and getting them to the polls to cast ballots on the day itself.
NATIONAL SECURITY and TAXES: On these two issues that inspire emotion among loyal Republicans and many swing voters, Republicans have long had an advantage, and President Bush and other prominent figures in the party will be pushing those messages hard.
INCUMBENCY: It is mathematically impossible for Democrats to win control of the House or Senate without beating a large number of current Republican officeholders. Incumbency brings many advantages, and lots of the Democratic challengers are inexperienced politicians who may collapse after a debate gaffe or an attack ad.
These advantages have benefited President Bush and his chief strategist, Karl Rove, in the past. And Republicans are now playing with maximum focus to limit their losses, rather than to win.
So before Democrats go predicting three weeks out that a takeover of Congress is inevitable, they would be wise to remember another warning from Berra.
It is still possible that the party of Howard Dean and Nancy Pelosi will experience "deja vu all over again."
Mark Halperin, the political director of ABC News, is co-author of the book "The Way to Win: Taking the White House in 2008" with John F. Harris of The Washington Post. Go to thewaytowin2008.com to find out more.