Democratic strategists acknowledged they are abandoning a dozen House seats the party now holds, as they try to salvage their majority in the chamber by shoring up candidates with better chances.

With Republicans expanding their advertising to broaden the field of competitive races, Democrats are shifting resources to help such senior lawmakers as House Budget Committee Chairman John Spratt (D., S.C.), and to head off Republicans in usually safe Massachusetts, where a southeast district that includes Cape Cod is competitive for the first time in decades…

The retrenching now under way happens in every election and is typically painful for the party running behind. Democrats are loath to admit that they are cutting off any of their candidates, but two Democrats familiar with the election strategy said that about a dozen seats now held by the party are lost. Ad-tracking data show that none of these districts is slated to receive party-funded TV ads.

“Any good strategy is about what you do, but also what you don’t do,” said Democratic strategist Mark Mellman, referring to the party’s lack of support for some candidates. “If you’re playing defense, you want to focus intensely on those seats where you can make a difference.”…

At the same time, Democrats are facing new efforts by Republicans to put more seats in play. The GOP House committee began airing ads recently against Democratic Rep. Charlie Wilson, who represents an eastern Ohio district that had been considered safe. Democrats followed and put up ads of their own.