Factbox: Senate poll averages by Real Clear Politics

(Reuters) - Democrats appear likely to narrowly hold their Senate majority in November's congressional elections, although Republicans could make big gains, according to poll averages compiled by the website Real Clear Politics.

Republicans must sweep nearly all of 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 six seats. That would leave Democrats with a narrow 52-47 edge, with independent Charlie Crist winning in Florida.

In the House of Representatives, the Real Clear Politics averages show an essentially even battle. Democrats lead for 202 seats to 201 for Republicans, with 32 contests considered toss-ups in the battle for a 218-seat majority.

In the generic congressional ballot, the latest Reuters/Ipsos national poll showed registered voters preferred the Republican candidate over the Democrat in their House district by 46 percent to 44 percent.

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) - Democrat Barbara Boxer leads Republican Carly Fiorina by 4.4 percentage points.

* Colorado (Democrat) - Although a primary has not been held to choose candidates, both likely Republican contenders have narrow leads on Democratic Senator Michael Bennet and his primary opponent Andrew Romanoff.

* Florida (Republican) - Independent Charlie Crist leads Republican Marco Rubio by 3.6 percentage points, with Democrats Kendrick Meek and Jeff Greene -- who meet in an August primary -- trailing badly.

* Illinois (Democrat) - Republican Mark Kirk leads Democrat Alexi Giannoulias by 1.7 percentage points.

* Kentucky (Republican) - Republican Rand Paul leads Democrat Jack Conway by 3.6 percentage points.

* Missouri (Republican) - Republican Roy Blunt leads Democrat Robin Carnahan by 5.3 percentage points.

* Nevada (Democrat) - Democrat Harry Reid leads Republican Sharon Angle by 3.7 percentage points.

* New Hampshire (Republican) - No primary yet, but Republican favorite Kelly Ayotte is ahead of Democrat Paul Hodes by 7.6 percentage points.

* Ohio (Republican) - Republican Rob Portman and Democrat Lee Fisher are essentially tied, with Portman holding a 0.7 percentage point lead.

* Pennsylvania (Democrat) - Republican Pat Toomey leads Democrat Joe Sestak by 2.3 percentage points.

* Washington (Democrat) - No primary yet, but incumbent Democrat Patty Murray leads favored Republican candidate Dino Rossi by 2 percentage points.

* Wisconsin (Democrat) - Democrat Russ Feingold leads likely Republican Ron Johnson by 2 percentage points.

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

(Compiled by John Whitesides in Washington; Editing by David Storey)