In 1996, Bill Clinton became the sixth president of the 20th century to win re-election. But two years later, he and his Democratic Party achieved something far more unusual by gaining House seats in a midterm election.

It was only the third time in the 20th century that the party in the White House did so. It happened under Republican Theodore Roosevelt in 1902 and Democrat Franklin D. Roosevelt in 1934. In every other midterm election, the party holding the White House lost House seats.

The 1998 result also marked the second straight midterm election that violated the normal pattern in which the party in the White House loses fewer seats at its first midterm than in its second midterm. Four years earlier, Clinton and the Democrats suffered an unprecedented first-midterm defeat by losing 54 seats and their 50-year-old House majority.

In November, in the first midterm election of the 21st century, we are going to find out if those unprecedented results during the Clinton years were as unique as his presidency or if the traditional off-year pattern will deprive George W. Bush of the House majority that has been crucial to his legislative successes.

Until 1994, that pattern had been firmly established. In the six prior midterm elections since World War II that took place two years after a new president was elected, the party out of power gained House seats every time, averaging a gain of 14 seats per election. And in four midterm elections in the sixth year of a presidency, the party out of power averaged gains of 37 seats.

The pattern hasn’t been as consistent in Senate elections, where the results depend as much on the strengths and weaknesses of individual candidates as on national trends. In six first-term midterm elections from 1954 through 1990, the party out of power gained Senate seats only three times. In the other three instances, the party in power gained seats.

Since the Democrats already control the Senate, they only have to break even this fall to keep their grip on its committees and its agenda.

In the House, Democrats need to gain seven seats to regain a majority, regardless of whether Rep. Ralph Hall, D-Tex., votes for Democratic leader Dick Gephardt as the speaker. Hall, who often votes with the Republicans, says he will vote for the most conservative candidate for speaker, and that is unlikely to be Gephardt.

Democratic strategists were optimistic about their prospects even before the recent concern over corporate abuses, the drop in the stock market and the state of the economy. One reason: They think they have minimized Republican gains from the post-census redistricting.

But GOP leaders believe they will keep House control. They expect gains from the redrawing of congressional districts and because normally Democratic industrial states lost seats while more Republican areas in the South and West gained.

While winning both houses is the Democratic goal, history says it might not be the best thing for them, especially if they hope to regain the White House in 2004.

Since World War II, there have been three midterm elections in which one or the other party regained control of both houses of Congress the Republicans in 1946, the Democrats in 1954 and the Republicans in 1994. In each instance, the winning party failed to capture the presidency two years later. The last time a party regained a single house was when the Democrats took the Senate in 1986. They, too, lost the next presidential election.

One reason: The loss of Congress enabled presidents from Harry Truman to Bill Clinton to blame the opposition party for their failure to get much done. That is the script President Bush is likely to follow if Democrats win the House and Senate this fall.

Carl P. Leubsdorf is Washington bureau chief of the Dallas Morning News.