Is the US becoming a one-party state? You might think so, from the silence of the Democratic party. Democrats have an opportunity to outline an alternative to the Bush administration's radical vision of an imperial America reshaping the Middle East in a series of pre-emptive wars. Instead, leading Democrats have limited their criticism to matters of process, pressing the president to consult Congress and work with the United Nations. Tom Daschle, the Senate majority leader, was typically indirect when, rather than denouncing White House policy, he criticised the president for using war for electoral advantage. With the exception of Al Gore, who last week spoke out against George W. Bush's rush to war, the opposition has forgotten how to oppose.

The near silence of the Democrats on foreign policy is caused not by timidity but by political paralysis. It can be attributed in part to the nature of the constitution. In a bicameral Congress no single figure speaks for the legislative branch. The president, unlike a prime minister or chancellor, is not beholden to the legislature. The fact that Mr Bush was elected with a narrow majority in the electoral college but lost the popular vote does not matter. The weakest US president is more visible and influential than the strongest congressional leader. That the presidency is the focal point of the American political system explains why Bill Clinton, the former president, and Al Gore, a presidential candidate, command more attention than Mr Daschle or Richard Gephardt, the House minority leader.

The weakness of the Democratic opposition is compounded by internal divisions. In foreign policy, black Americans, the most important Democratic voting bloc, tend to be anti-interventionist and to side with the Palestinians. Jewish Americans, who provide the Democratic party with many of its campaign donations, are increasingly hawkish where threats to Israel are concerned. Regional differences are just as important. White southern Democrats tend to be as hawkish as white northern Democrats are dovish. In the words of Will Rogers, the US satirist of the early 20th century: "I belong to no organised political party. I am a Democrat."

Although they make up only about a third of the electorate, partisan Republicans are far more unified than their Democratic adversaries. The Republican party today is dominated by conservative white southern Protestants while the Democrats are an unstable coalition of ethnic groups.

The near-parity of the two parties in the federal government is another cause of Democrat paralysis. The presidential race in 2000 was a contested photo finish. The Republicans control the House by a tiny margin and the Democrats control the Senate by one seat. This explains the surprising withdrawal, under pressure from party leaders, of Robert Torricelli, the disgraced New Jersey senator. "I will not be responsible for the loss of the Democratic majority in the United States Senate," he explained.

Determined to hold on to the Senate, and to recapture the House of Representatives, the Democratic leadership has assured Mr Bush of congressional authorisation for a new war against Iraq. Democratic strategists believe the president is vulnerable on domestic issues but not in foreign policy. Mr Bush benefits from the irrational bloodlust of a shocked American public that was not sated by the toppling of the Taliban regime in Afghanistan. The Democrats' eagerness to fight the mid-term congressional elections on issues such as the Enron scandal and opposition to the privatisation of Social Security explains why so many want to get a resolution out of the way. The sooner that the Democrats can change the subject to domestic policy, the better.

Even in more favourable political circumstances, members of Congress are wary about opposing presidents bent on war. If the war fails, the blame falls on the president. If the war is a success, or can be portrayed as such, the legislator can brag about having voted to authorise it. During his one term as a member of Congress, Abraham Lincoln infuriated many of his Illinois constituents when he opposed President Polk's war with Mexico. In later years, Lincoln would tell the story of another member of Congress, who said he had voted against one war and would never do so again: "From now on, I'll vote for war, pestilence and famine." So far, Mr Bush has not asked Congress for resolutions authorising pestilence and famine but if he were to do so a majority of Democrats might well vote in favour.