By Max Deveson
BBC News, Washington
"It doesn't get bigger than this."
Congress wields tremendous power over domestic policy
That is what Larry Sabato, Professor of Political Science at the University of Virginia, is saying about the potential for big Democratic gains in the US Senate races in November.
One-third of Senators are up for election every two years, as well as all Congressmen, and of this year's 35, the majority are Republicans.
Controlling Congress is the key to any President getting through his domestic policy agenda.
In 2006, the dramatic mid-term elections saw Democrats regaining control of both the House of Representatives and the Senate.
But they currently only have a slim majority of 51-49 in the Senate (and one of these votes is the independent Democrat Joe Lieberman who is campaigning for Senator McCain).
POSSIBLE DEMOCRATIC GAINS
Key Senate races
Alaska, Colorado, Georgia, Kentucky, Minnesota, Mississippi, New Hampshire, New Mexico, North Carolina, Oregon, Virginia
This year, the Democrats are poised to build on their 2006 success by winning even more seats, even, perhaps, the 60 needed for a filibuster-proof majority, which would stop Republican lawmakers "talking out" legislation with which they disagree.
This would allow Democrats in the Senate to push through a much more ambitious - and liberal - agenda, especially if the American people also elect Barack Obama to the White House in November.
The opinion polls nationally are running strongly towards the Democrats.
But to make major gains in the Senate, Democrats would have to pick up seats in some very unlikely states.
Republicans have held Alaska's Senate seats since 1981
One of the most surprising is Alaska, whose Senate delegation has been solidly Republican since 1981.
Incumbent Republican Senator Ted Stevens, who faces re-election in November, has been a dominant figure in Alaskan politics since 1968, when he first won his seat.
But his grip on power is beginning to look decidedly shaky - he is currently on trial, charged with covering up renovation work carried out on his house free of charge by oil company employees.
Even before his indictment was served, Mr Stevens had been facing a tough re-election battle against the Democratic Mayor of Anchorage, Mark Begich.
None of the other Senate races have - as yet - been affected by scandal.
But four other Republican-held seats where sitting Republicans are retiring are looking like possible pick-ups for the Democrats.
Of these four, the two most likely openings for the Democrats are in Virginia and Colorado, two traditionally Republican states that have been increasingly voting for Democrats in recent elections.
In Virginia, the seat of outgoing Republican John Warner - Elizabeth Taylor's ex-husband, no less - is being fought over by two former Virginia governors: Republican Jim Gilmore and Democrat Mark Warner.
Mark Warner has been credited with beginning the wave of recent Democratic victories in the state that culminated with election wins for current Governor Tim Kaine and Senator Jim Webb in 2006.
Polls suggest he is streets ahead of Mr Gilmore.
In Colorado, where the Democrats won a Senate seat in 2006, Congressman Mark Udall, a Democrat, is leading former congressman Bob Schaffer.
Like Virginia, the trend in Colorado has been towards the Democrats of late, and both states are showing signs of backing Barack Obama in the presidential poll, which should help Democratic candidates lower down the ballot.
The Democrats also have good chances in another open seat.
Tom Udall - and his cousin Mark - are both running for the Senate
In New Mexico, Democrat Tom Udall - a cousin of Mark Udall - is challenging his fellow congressman, Republican Steve Pearce for outgoing Senator Pete Domenici's seat.
The Udalls appear to have luck on their side this year.
Tom Udall's opponent could have been the other member of New Mexico's congressional delegation, moderate Republican Heather Wilson.
But Ms Wilson lost out to Mr Pearce - a staunch conservative - in the selection primary, and Mr Udall will not be forced to fight so hard for centrist voters. Polls suggest he will beat Mr Pearce in November.
The Udalls are a Western dynasty. Tom's father, Stewart Udall, was an Arizona Congressman and Secretary of the Interior a generation ago under John F Kennedy.
The fourth open seat being talked about as a possible Democratic gain is - perhaps surprisingly - Mississippi.
Republicans like Senator Liz Dole are facing real challenges
The state has long been a Republican stronghold, but - without the advantage of incumbency - Republican candidate Roger Wicker, who was appointed to fill out the remainder of Trent Lott's Senate term, is having a tough time defending it against the challenge of Democrat - and former Mississippi governor - Ronnie Musgrove.
Mr Musgrove will be hoping that the presence of Barack Obama at the top of the ticket will encourage a large turnout among African-American voters, which would improve his chances of success.
The Republicans are also facing strong challenges in traditionally Republican states like North Carolina - where Democrat Kay Hagan is currently leading Republican incumbent Elizabeth Dole in opinion polls - and Georgia, where polls suggest the two parties are neck and neck.
Polling also indicates that even the leader of the Republicans in the Senate - Mitch McConnell - has a fight on his hands to keep his ultra-safe Kentucky seat.
The Democrats also have their eye on a number of seats held by Republicans battling for re-election in states that are traditionally Democratic.
Of these, the most likely Democratic gain would be in New Hampshire, where incumbent Republican John Sununu faces a very tough fight against popular former state governor Jeanne Shaheen.
Two other Republican incumbents in broadly Democratic states - Gordon Smith in Oregon and Norm Coleman in Minnesota - also face the prospect of losing their seats to strong Democratic challengers.
Mr Coleman's battle against Democrat Al Franken, a comedian and radio host, is complicated further by the presence of a third party candidate, Dean Barkley, who briefly served in the Senate, on the ballot.
"No-one knows what's going to happen in Minnesota," says Professor Sabato.
Glimmer of hope
As the races currently stand, the Democrats have a good chance of gaining at least seven seats - Alaska, Colorado, New Mexico, New Hampshire, North Carolina, Oregon and Virginia - and could pick up as many as eleven.
Seven pick-ups would give them 58 seats in the senate, while eleven would put them on 62.
Why does any of this matter?
Well, because of the Senate's procedural rules, it takes 60 votes to overturn a "filibuster" - when a senator "talks out" a bill, effectively killing it.
So even if a party has only a minority of seats in the senate, it can block the passage of legislation with a mere 41 votes.
Indeed, since winning a Senate majority in 2006, the Democrats have been consistently stymied by Republican filibusters.
The more Senators the Democrats have, the greater their chances of pushing through their legislative agenda in the face of Republican opposition.
Professor Sabato reckons that if the Democrats win 58 seats, the votes of the handful of moderate Republicans (like Olympia Snowe of Maine, or Arlen Specter of Pennsylvania) will in fact be enough to allow Barack Obama - if elected - to get his legislative agenda through the Senate.
So when you are watching the results come in on election night, make sure you keep an eye on the Senate races as well.
If the Democrats reach the magic number, they could be in a position to override Republican opposition and introduce major new legislation.