By Paul Reynolds
World Affairs Correspondent, BBC News website
The issue for this year's mid-term elections in the United States is whether the Republicans can hold onto their control of both the Senate and the House of Representatives.
The Republicans' domination has prevailed in the House since their major victory in the 1994 elections, which heralded the start of the "Republican revolution".
The Republicans control the Senate and House of Representatives
It was their first majority in the House of Representatives since 1954.
They also won the Senate in 1994 and although their power in the Senate has not been unbroken since then, they have certainly had the best of it.
Shift to the Democrats?
This year, the Democrats hope the difficulties President Bush is facing, combined with a possible "time for a change" mood, will give them opportunities.
CONGRESS BALANCE OF POWER
HOUSE OF REPRESENTATIVES:
435 seats - all to be contested in mid-terms
Republicans hold 231 seats; Democrats 201; one independent; two seats vacant
Democrats need to win net 15 seats to win control of House
100 seats - 33 to be contested in mid-terms
Republicans hold 55 seats; Democrats 44; one independent
Democrats need to win net six seats to win control of Senate
They are helped by the fact that the president's party often loses seats in the mid-terms.
However, because of a number of factors - including the power of incumbents, the redistribution of House seats following the 2000 census and local loyalties to a particular member regardless of party affiliation - it will require a significant shift of public opinion for the Democrats to succeed.
Whether the Democrats can get the country behind them is the question. Or will the conservative mood in large parts of the United States prevail?
A shift to the Democrats in the mid-terms would also give them hopes for success in the Presidential election in 2008.
The elections, on Tuesday 7 November, are for the whole House (435 seats) and one third of the Senate (33 out of 100 seats).
The system is designed to allow for a snapshot expression of views in the House, whose members are all up for re-election every two years, but for the Senate to be protected against this.
The overlapping nature of the US electoral process is part of the deliberate decision made by the writers of the Constitution to spread power widely.
There are also 36 races for state governorships which are often keenly watched to see if they throw up likely presidential candidates.
Currently the Republicans hold the House of Representatives with 231 seats against 201 for the Democrats. There are two vacant seats and one independent member.
In the Senate the figures are 55 for the Republicans and 44 for the Democrats with one independent.
"The Republicans still look pretty strong," said Professor Philip John Davies of de Montfort University and the Eccles Centre for American Studies at the British Library.
When the plates do shift, they herald major changes in US society
"They have taken steps to defend themselves in the redistribution of House seats and were helped in this by their control of key governorships. That is part of the political game in the US.
"You also have to remember that incumbents have a huge advantage. A big swing could overcome that but I think that the Republicans will hold onto both the Senate and the House, though with some losses," he said.
"I am not expecting a shift of the plates," he added. "If it happens, it will be something remarkable."
Among the Senate races generating interest are:
Tom DeLay is running to keep his House seat in district 22 in Texas
- Florida: Bill Nelson is the only Southern Democrat to face re-election. Congresswoman Katherine Harris, of the 2000 "hanging chad" fame, might win the Republican nomination.
- Washington: Democrat Maria Cantwell, who supported the Iraq war, is running for re-election. What effect will Iraq have?
- Connecticut: Democrat Joe Lieberman, still an Iraq war supporter, also faces the Iraq issue.
- Vermont: Jim Jeffords, who switched from the Republicans to become an Independent, is retiring. A Democrat-supporting independent might take over.
- Maryland: Veteran Democrat Paul Sarbanes is retiring.
- Pennsylvania: Republican Rick Santorum is up for re-election in a test of his conservative views.
The effect of the Jack Abramoff lobbying scandal might be felt in Montana where Republican Conrad Burns - who had links to Abramoff - is standing again.
Note also Hillary Rodham Clinton's race for re-election in New York, Edward Kennedy's easy run at an eighth term and elder statesman Robert C Byrd's effort to get a ninth term in West Virginia.
Arnold Schwarzenegger is eyeing his first full term
In the House races, most eyes are on Republican Tom DeLay who had to step aside as Majority Leader to face a corruption charge.
In the governorships, Jeb Bush cannot stand again in Florida because of a term limit. Mitt Romney has chosen not to stand in Massachusetts. Look out for a possible Romney run for the Republican presidential nomination in 08.
George Pataki is vacating the New York spot. And Arnold Schwarzenegger is running for his first full term, having been elected in the special recall election in 2003.
Mid-term elections often produce little of major interest, given the fragmented nature of the system and the vagaries of the votes.
But when the plates do shift, they herald major changes in US society.
The 1994 Republican landslide has been noted. Its influence is still felt (though it did not prevent the re-election of the Democrat Bill Clinton as president two years later. The US voters sometimes like to have their cake and eat it).
It was the other way around in 1930. In 1928 Herbert Hoover, who had been the hero of the American relief effort in Europe after the First World War, had been elected president with a huge majority.
Yet in 1930, a year after the Great Crash, he was seen as a liability and the Democrats made major gains in the mid-terms. The era of the New Deal began. In 1932, Roosevelt became president.