To understand how this story began, you need to focus on this line in Everett Herald's scoop:

This information originally came to light through the sleuthing of the Democratic Senate Campaign Committee. The political operation recently dispatched a team of researchers into Snohomish County to comb through mounds of court and assessor records on Rossi and [his employer] Coast Equity Partners.

It's all part of a smart strategy that I wrote about here—essentially calling Rossi's "maybe I'm running for Senate, maybe I'm not" bluff by just assuming he's in and tearing him down before he's formally declared.

Now to today's hit from the DSCC, as described by The Atlantic online:

The investment firm in which Rossi is a minority partner owes $20,000 in unpaid taxes on a business property in Everett, Washington, according to tax records from Snohomish County. Coast Equity Partners did not pay taxes, due in October 2009, on a multi-unit property that was rented to a motor sports dealership. (The sum includes roughly $18,000 in taxes and $2,500 in fees and penalties for delinquency.)

The firm says the unpaid taxes are "nothing Dino is involved in," and that may well be true, but the potential political hit is there ("Rossi associates with tax cheats") as is the (continuing) message to Rossi from DSCC: get into this race and we're going to come at you hammer and tongs.