From Ad Age:

Kevin Kosar, a VP of policy at the Washington, D.C.-based R Street Institute, says any disruption for advertisers would be contingent on whether the appropriations bills — which provide funding for various government departments — are expired. The U.S. Postal Service, which works with IPG’s MRM/McCann, is a self-funding agency and is thus not affected by shutdowns, he says. MRM declined to comment.

When a department’s appropriation is expired, Kosar says federal agencies can run into the issues of manpower, since the agencies don’t have the funds to pay employees, and the approval of contracts — since federal agencies can’t enter into a new contract with a contractor such as an advertising firm when they don’t have the approved funding.

An advertiser may also run into trouble with the execution of campaigns if its federal agency client has an expired appropriation.

“Certain steps may get gummed up, they may be stuck in limbo,” he says.