finds it hard to get his own way. Between 1953 and 1996, only 46 per cent of proposals submitted by presidents were passed into law.

How else does Congress limit him?

Although the Constitution grants him some leeway to begin a sudden, defensive war, he still in theory depends on Congress to do so (see 'Vietnam', below). Congress's real power over the White House, however, lies in its control of the purse strings - "the most complete and effectual weapon with which any constitution can arm the immediate representatives of the people", as 'father' of the Constitution James Madison put it. And so it has often proved. In the mid-1990s, the federal government shut down for 28 days when the Republican-controlled Congress refused to approve Bill Clinton's budget.

And what about impeachment?

The House of Representatives, by a simple majority, can vote to impeach a president (ie. subject him to trial by the Senate) for "Treason, Bribery, or other high Crimes and Misdemeanours". But though three have faced impeachment proceedings - Clinton, Nixon and Andrew Johnson - none has been sacked. Nixon resigned before the House could vote; Clinton survived his trial. Johnson, who succeeded Abraham Lincoln after his assassination, was dubbed 'King Andy' for his resistance to Congress. In 1866, for instance, he vetoed the Civil Rights Bill giving freedom to the slaves. Desperately unpopular (Congress overrode his vetoes 15 times) he survived impeachment by a single vote.

Is the Supreme Court easier for presidents to control?

The president's power to nominate judges gives him scope to shape the politics of the court. But it doesn't always work out that way. The Senate can reject appointments; and in any case, once installed, judges are hard to control. George Bush Snr's appointment, David Souter, ended up being one of four justices to side against his son in the Florida recount of 2000.

How do they get anything done?

By ignoring the rules: most great presidents have become so while acting outside their authority. Jefferson carried out the Louisiana Purchase (buying the land of 15 states from Napoleon) despite having no constitutional right to spend $23m. Lincoln suspended the right of habeas corpus during the American civil war. In the 20th century, President Theodore Roosevelt pioneered the use of Executive Orders, presidential decrees that have the force of law. They have since been used to desegregate the US Army (Truman), ban the use of federal funds for abortion (Reagan) and start an air war in Kosovo (Clinton). As Paul Begala, one of Clinton's advisers, said: "Stroke of the pen. Law of the land. Kinda cool."

Vietnam, Iraq and the White House

"Congress shall have power to... declare War," reads Article I, section 8 of the Constitution. And so it has done five times in US history. That just leaves about 120 other occasions when the president has sent US troops into battle. Like the UK, America has largely stopped declaring its wars. The president instead asks for an "authorisation of force" from Congress, as in the case of the Vietnam war - in which 58,000 Americans and up to four million Vietnamese died; or in the case of the Korean war, no authorisation at all. After the Vietnam war, Congress tried to re-assert its authority with the War Powers Resolution of 1973, which would force the president to consult Congress and regularly seek re-authorisation for an ongoing conflict. But every president since Richard Nixon (who tried to veto it) has ignored it. The White House is still fighting the war in Iraq under an authorisation to remove (the long dead) Saddam Hussein. But now, two former secretaries of state, James Baker and Warren Christopher, are proposing a new law which would oblige the president to consult a special congressional committee before any "significant armed conflict", and to get congressional approval for any operation lasting longer than 30 days.