But much depends on the definition of "landslide". Rollins did not mean a result similar to 1984, when Reagan won virtually every state. What he meant was that Obama's popularity might be enough not only to
win the presidency, but also to carry both houses of Congress. To some, that would suggest absolute power.

If polls are correct, the Democrats should easily retain control of both houses of Congress. They might even be able to reach the magical figure of 60 seats in the Senate, allowing them to prevent the Republicans from blocking legislation through filibuster. But, despite the alarmist talk from the McCain camp, such a result would not mean that Obama's power will be limitless. In contrast to Britain, where a healthy parliamentary majority renders a prime minister virtually unassailable, an American president can never take Congress for granted.

Two events of recent weeks illustrate this point. Two weeks ago, General Colin Powell, who was once touted as a potential Republican president, pledged his support for Obama. What is important is that he did so without formally defecting from the Republican Party. In other words, party loyalties are always weak in America. Individuals weigh a wide variety of factors in deciding whom to support. Party identification is never paramount. That fact was never more strikingly evident than in Bush's struggle to get his $700bn bailout bill through Congress. Those who consistently wrecked the bill were Republicans. If not for Democratic support, Bush would still be wringing his hands.

Under the constitution, the president is the chief executive. He does not make laws, he merely executes them. Matters involving the budget, social policy and law require Congressional co-operation. The president might indicate a desire for a certain raft of reforms, but it is up to Congress to legislate those reforms.

It is obviously advantageous if a president has 'control' of both houses of Congress, but 'control' does not mean that Congress will always act according to the president's whim. For instance, Clinton's victory in 1992 coincided with Democrats winning both houses of Congress. Clinton quickly discovered, however, that a Democrat from liberal Massachusetts and a Democrat from conservative Georgia are two very different beasts.

Ironically, Ronald Reagan, who did not enjoy the luxury of a Republican Congress, had much better success at getting legislators to bend to his will with Democrats in the south, which proved crucial. Reagan's conservative social agenda and his strident patriotism harmonised with the sentiments of southern voters. Democratic congressmen therefore felt inclined to support Reagan, if only for reasons of self-preservation. A similar, if less profound, effect was evident nationwide. Realising the immense personal popularity of Reagan, Democratic legislators were reluctant to thwart his pet projects, since they did not want to risk losing support in their own districts. Thus, Democrats facilitated the Reagan revolution.

One suspects that Obama, if he wins, will find that his Congress behaves more closely to that of Clinton than that of Reagan. Though he might have 'control', he will not control. Excessively liberal legislation will alienate Democrats in the south.

There are, however, two jokers in the pack. The first is the extent to which the electorate and Congress are taken in by the rhetoric of change. If the country genuinely believes that the 2008 election is a watershed moment, the country might will it to be so. Congress might, in turn, be swept along by the consensus for change. A similar situation occurred in 1960, when the country expected "new frontiers". Though John Kennedy won by a tiny margin, once elected, he enjoyed a massive majority willing him to succeed. Congress, in tune to the national mood, generally supported the Kennedy agenda.

The second joker is the economic crisis. Because comparisons to the Great Depression are inevitably made, it is possible that Obama will enjoy the same kind of support that Franklin Roosevelt encountered after his election in 1932. In times of crisis, the country and Congress tend to rally behind the president. Thus, the deeper the crisis gets, the more loyal Congress might prove to be. That said, however, America of today is very different from that of 1932. Consensus is now difficult to achieve and virtually impossible to maintain.

One other important factor bears consideration. If Obama wins, it will be the first time since 1960 that an election has produced both a president and a vice-president who are former senators. That is hugely important, since Congress responds well to those who understand how the system works. While Kennedy was not particularly popular in Congress, he could rely upon the power of Lyndon Johnson to get things done. A Johnson-type role might be in store for Joe Biden.

All of this illustrates a central fact of American politics, namely that the person is more important than the party. No matter how supportive of an Obama presidency Congress might be, the actual support that Obama enjoys will be determined by his ability to convince legislators that he deserves their trust. As the last few weeks have demonstrated, legislators tend to be contemptuous of presidents who seem unworthy of respect.

• Gerard J DeGroot is Professor of Modern History at the University of St Andrews