Republican Sen. David Vitter doesn't have an announced opponent yet but it's clear he's expecting a challenge from Rep. Charlie Melancon, a southeast Louisiana Democrat who has said he's considering the race.

In ads critical of Democrat-backed health care plans, Vitter says Melancon supports those plans. Meanwhile, state Republicans have taken aim at Melancon for taking a taxpayer-funded trip last year that included Antarctica, Australia and Hawaii.

Melancon's camp notes that Melancon voted against the Democratic health care plan in a House committee. And it defends last year's trip as a legitimate House Science and Technology Committee trip to study climate change issues. State Democrats, meanwhile, have launched a new Web site critical of Vitter.