(CNN) All of the Democratic candidates in more than two dozen of the most competitive House races outpaced their Republican rivals in recent campaign fundraising, underscoring the Democrats' enormous financial advantage heading into the midterm elections, a CNN analysis of newly filed campaign-finance reports shows.

Across all the 28 races that CNN defines as toss-ups, Democratic candidates raised nearly $69.6 million during the July-to-September fundraising quarter, more than three times the $21.4 million collected by Republicans in those contests, the tally shows.

Those figures underscore how difficult the terrain has become for House Republicans in the midterms that are now just three weeks away and how much the party is relying on major financial benefactors, such as Las Vegas casino magnate Sheldon Adelson, to remain competitive in the homestretch to Election Day.

"The fundraising is an indication of the off-the-charts intensity that Democrats are experiencing," said Republican strategist Ken Spain, a former top staffer at the House Republican campaign committee. "Clearly, Republicans face an uphill battle."

Democrats need to flip 23 GOP-controlled seats to seize control of the chamber. Republicans, meanwhile, are grappling with a near-record number of retirements from the House and tradition: A president typically loses seats in Congress in his first midterm election.

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