Later today, Americans will vote for both houses of Congress in an election that could dramatically reduce the power of the Bush presidency — with major consequences  for the rest of the world.

The US House of Representatives has 435 members elected for two-year terms. Most seats are safe: of the 232 currently Republican-held, for example, 121 (more than half) have margins greater than 16% (compare our House of Reps, where only 13 of the 87 Coalition seats are that safe).

Control of the lower house is therefore very hard to shift; the Republicans have held it since 1994, and the Democrats held it continuously for 40 years before that.

This year the Democrats look almost certain to get it back: they need a uniform two-party swing of 5.3% to win the necessary 15 seats, and most of the polls have been showing well over that. Online betting exchange Tradesports is quoting the Republicans this morning at about 3-1 against. (Past official results are available here; Wikipedia has a good summary, with links.)

The Senate is the more interesting house. There are 100 senators (two per state), but they serve six-year terms, with a third retiring every two years. The Republicans currently have a 55-45 majority, so the Democrats need to make six gains out of the 33 up for election. (A 50-50 tie would leave the Republicans in control, because vice-president Dick Cheney has a casting vote.)

At this stage the Democrats look like holding all of their Senate seats (although there’s some doubt about New Jersey) and picking up another three: Ohio, Pennsylvania and Rhode Island. Possible extra gains, roughly in order of probability, include Montana, Missouri, Virginia, Tennessee and Arizona.

Tradesports is quoting about 2-1 against the Democrats getting the necessary Senate gains, although odds for the individual races show Democrats ahead in enough for a bare majority. The same goes for the opinion polls summarised at Slate’s election watch. Montana, Missouri and Virginia are all too close to call, but with Democrats ahead in each.

If they do pick up the required seats, Democrat Senate control could still be tenuous, because it would depend on two independents: socialist Bernie Sanders, who will win comfortably in Vermont, and ex-Democrat pro-war incumbent Joe Lieberman, who looks like being returned in Connecticut. Both have promised to caucus with the Democrats, but senators are notorious for their resistance to party control.