Pittsburgh's only hot Democratic primary is being fueled in large part with out-of-town cash, but rumored Republican interference hasn't yet materialized. The battle between state Rep. Michael Diven and challenger Rich Nerone - both Brookline Democrats - has caught fire because many top state and local Dems want to get rid of the freewheeling incumbent. (See Political Footballs: "Rabbit Punch," April 14.) Democratic claims that Diven is being bankrolled by the Republicans aren't borne out in April 16 campaign expense filings, which list contributions from Jan. 1 through April 12. Diven's biggest contributors, each kicking in $10,000, were the campaign committees of Pittsburgh Council President Gene Ricciardi and Lt. Gov. Catherine Baker Knoll (both Democrats), and a Philadelphia political committee called Citizens for Government Reform that backs members of both parties. Diven's concern that House Democratic leaders would funnel $100,000 or more to Nerone has also proved unfounded so far. Nerone's biggest campaign donor was the Pittsburgh Fire Fighters union, giving $2,500. The race features dueling junk-mail and cable-TV ads, thanks in large part to people who don't have to endure them. Of the $79,027 Diven raised, $39,450 came from people whose zip codes don't start with Pittsburgh's 152. Of that, $7,100 came from out of state. Nerone raised $35,010, including $13,200 from outside of Pittsburgh and immediate environs. Of that, $4,950 came from out of state. That's probably not the whole story. Concealing controversial contributions until after the election is de rigueur in Pennsylvania politics, and there'll likely be cash flowing in right up to the April 27 vote.