US elections 2010: The basics

President Barack Obama's Democratic Party won a big majority in the two houses of Congress in 2008, but could lose control of one or both in mid-term elections on 2 November.

The Republican Party needs to take 39 seats from the Democrats to gain a majority in the House of Representatives - and forecasts from four leading experts suggest they will easily succeed. None of the experts expect the Republicans to take control of the Senate.

We have aggregated their forecasts to produce a sketch of how the Congress could look after the election.

At present the Democrats control 59 seats in the Senate, if you include two independents who usually vote with them. That's a majority of nine (51 seats would be a majority of one). They have 256 seats in the Senate, if you include one vacant seat that was most recently occupied by a Democrat, which is a majority of 39 (218 seats would be a majority of one).

The aggregated panel prediction above is produced by averaging the net Republican gains forecast by the experts, who have been chosen because they are highly respected and keep their projected figures for the seat-breakdown in the next Congress regularly updated. The experts are:

Charlie Cook - The Cook Political Report

Stuart Rothenberg - The Rothenberg Political Report

Larry Sabato - Sabato's Crystal Ball

Nate Silver - FiveThirtyEight Blog, New York Times

Predicted Republican net gains

Cook Rothenberg Sabato Silver

House

50-60

55-65

55

53.1

Senate

6-8

6-8

8

7.3

Where a range is given, we take the mid-point for the aggregated forecast.

Of two vacant seats in the House of Representatives, one was most recently held by a Democrat, the other by a Republican. This is why it is usually said that the Republicans need to gain 39 seats to achieve a majority - even though they are currently 40 seats short of the magic number 218.

While the two independents in the Senate currently caucus with the Democrats, and usually vote with them, there has been some speculation that one them, Joe Lieberman, could decide to caucus with the Republicans if they do well in this election.

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