In the wake of Doug Hoffman's withdrawal from the House race in New York's 23rd district, Democrats are launching an advertising onslaught against Republican Matt Doheny.

In the last 24 hours, the Democratic Congressional Campaign Committee layered another week on its existing TV ad buy in the district, according to a Republican strategist who tracks Democratic ad buys. The committee will now be up on the air in the district from Oct. 5 through Election Day. Its ads were originally set to start Oct. 12 in the Watertown media market.

Its first ad says Doheny "funded an organization pushing unfair trade — like NAFTA."

"Worse, Doheny signed a pledge that protected tax breaks for companies shipping jobs overseas," the announcer says in the spot. That claim refers to Doheny's support for the Americans for Tax Reform pledge, which says signatories will "oppose any net reduction or elimination of deductions and credits unless matched dollar for dollar by further reducing tax rates."

Democrats want to change the current tax law to prevent corporations from deferring taxes on their foreign income, which business groups and Republicans say amounts to a tax increase.

Rep. Bill Owens (D-N.Y.) has also used Doheny's support for the pledge as a political cudgel against his challenger.

Republican sources say the DCCC has significantly increased its ad buy from some $250,000 in the Albany, Syracuse, Watertown and Burlington, Vt. media markets.

Democrats worry that without Hoffman to split the conservative vote, Owens will have trouble holding the Republican-leaning district.

Owens won the November 2009 special election for the seat by only some 3,500 votes — and that was with Republican Dede Scozzafava's name on the ballot.

The state lawmaker suspended her campaign shortly before the vote and said she was supporting Owens. But her name remained on the ballot, and Scozzafava ended up getting more than 8,000 votes — enough to cost Hoffman the election.

Hoffman dropped his bid for the seat earlier this week, but his name will still appear on the ballot on the Conservative line. He said Tuesday that he wants his supporters to vote for Doheny.

--Updated at 4:27 p.m.