Reversing a pattern that has lasted almost a century, the Republicans have won congressional seats in a midterm election, and taken control of the Senate. Part of the reason is President Bush's personal popularity. Part of the reason is 9-11. But there are also deeper trends at work.

America has a one-party system of government. What this means is that one party tends to dominate American politics in a given era. The major party sets the agenda, and the other party has the choice of reactively opposing that agenda, or of sounding a feeble cry of "me too."

During the Jackson era the Democrats were the majority party. This dominance lasted a half century, until the Civil War. After that war the Republicans became the majority party, a position they held until the Great Depression. Since 1932, the Democrats assumed the majority position, and they remained the majority party for most of the twentieth century.

Only in 1994, when the Republicans won both houses of Congress, did the Democrats lose their majority status. Since then, the big question has been: will the Republicans become the majority party? President Clinton's election and re-election placed that outcome in doubt. But the results of Tuesday's midterm election clearly show: the Republicans have now become America's majority party.

What this means is that President Bush will have the support he needs for a war against Iraq, and more generally for his war against terrorism. It means that the Bush tax cuts are likely to become permanent, and further tax cuts may be forthcoming. It means that Bush nominees are more likely to be confirmed to appeals courts, and, if there is an opening, to the Supreme Court. In short, Bush now has the political mandate that he lacked when he was first elected, and he will be judged in 2004 by what he does with it. More broadly, what the Republicans do over the next few years will determine whether their power endures, whether they become the majority party for the next few decades.

As the minority party, the Democrats will have the choice of resolutely opposing the Bush agenda, or of reluctantly accommodating it. This is exactly the choice that Republicans faced in the era of the New Deal, when the Democrats were riding high. At first Republicans threatened to repeal the New Deal, yet eventually they accommodated themselves to it, and even subsequent Republican presidents like Eisenhower, Nixon, and Ford ended up consolidating and even expanding the welfare state.

The left-wing of the Democratic party is sure to blame Tom Daschle and Richard Gephardt for refusing to oppose Bush on Iraq, and for refusing to call for a repeal of the Bush tax cut, and for refusing to make a bigger issue of corporate greed, and for generally keeping their distance from the progressive antiwar and redistributionist agenda. Yet if the Democrats move sharply to the left, they risk losing the centrist reputation that they gained during the Clinton years-a reputation that won Clinton two terms in office.

What is not clear is how Bush will use his Republican majority, something that Reagan never had, indeed something that no Republican has had for a very long time. Nor it is possible to say, at this early stage, if the Democrats will move left, move right, or begin a long period of fratricidal conflict. What is certain is that the fulcrum of American politics shifted on Tuesday, and the country will be living with the consequences for the next several years.