The Republican primary for the swing House seat in Oregon just keeps getting nastier -- a development that's cheering national Dems, because it improves the Democrats' chances of holding on to this key seat that could otherwise be at some risk of slipping into Republican hands.

Check out this vicious new attack ad that businessman Mike Erickson is airing -- it hits back at former state Rep. Kevin Mannix for spreading a two-year old e-mail that accused Erickson of impregnating a younger woman and paying for her abortion:

If things continue at this rate, the eventual Democratic nominee will be up against a badly bloodied GOP opponent, making it more likely that national Dems will hold the open seat.