FACTBOX-U.S. Senate poll averages by Real Clear Politics

WASHINGTON, Sept 2 (Reuters) - Democrats are struggling to hold their U.S. Senate majority in November's congressional elections, with Republicans headed for big gains, according to poll averages compiled by the website Real Clear Politics.

Republicans must sweep nearly all the competitive races to pick up the 10 seats needed for a majority in the Senate, where Democrats now hold a 59-41 edge.

The polls show Republicans leading in races that could give them a gain of eight seats, which would leave Democrats with a narrow 51-49 edge. In Washington state, Republican Dino Rossi moved ahead of incumbent Democratic Senator Patty Murray on Thursday in the poll averages.

In the House of Representatives, the Real Clear Politics averages show Republicans with a slight edge two months before the Nov. 2 elections. Republicans lead with 206 seats to 194 for Democrats, with 35 contests considered toss-ups in the battle for a 218-seat majority.

Republicans must pick up 39 Democratic seats to claim House control in November.

Here is a look at the Real Clear Politics poll averages in the most competitive Senate races, with the party currently holding the seat in parenthesis.

* California (Democrat) - Democratic Senator Barbara Boxer leads Republican challenger Carly Fiorina by 3.5 percentage points.

* Colorado (Democrat) - Republican Ken Buck leads Democratic Senator Michael Bennet by 2.5 percentage points.

* Florida (Republican) - Republican Marco Rubio leads independent Charlie Crist by 2.4 percentage points, with Democrat Kendrick Meek trailing badly.

* Illinois (Democrat) - Republican Mark Kirk leads Democrat Alexi Giannoulias by 1.4 percentage points.

* Kentucky (Republican) - Republican Rand Paul leads Democrat Jack Conway by 5.5 percentage points.

* Missouri (Republican) - Republican Roy Blunt leads Democrat Robin Carnahan by 6.7 percentage points.

* Nevada (Democrat) - Democratic Senate Leader Harry Reid leads Republican challenger Sharron Angle by 1.3 percentage points.

* New Hampshire (Republican) - Ahead of a Sept. 14 primary, Republican favorite Kelly Ayotte leads Democrat Paul Hodes by 8 percentage points.

* Ohio (Republican) - Republican Rob Portman leads Democrat Lee Fisher by 3.7 percentage points.

* Pennsylvania (Democrat) - Republican Pat Toomey leads Democrat Joe Sestak by 8.5 percentage points.

* Washington (Democrat) - Republican Dino Rossi leads Democratic Senator Patty Murray by 2.3 percentage points.

* Wisconsin (Democrat) - Democratic Senator Russ Feingold leads likely Republican challenger Ron Johnson by 1 percentage point.

Republicans also hold commanding leads in four Democratic-held states -- Arkansas, Delaware, Indiana and North Dakota.

(Compiled by John Whitesides in Washington; Editing by Doina Chiacu)