Two issue advocacy groups — one liberal, the other conservative — have taken to the airwaves to aid shaky Congressional incumbents.

Americans United for Change, a left-leaning group that has spent most of the election cycle attacking Republicans for the Iraq War and the poor economy, is set to launch a series of TV ads today to prop up vulnerable House Democrats.

The ads, timed to coincide with the delivery of economic stimulus rebate checks to taxpayers, praise the four Members for voting for the stimulus package and being part of a Democratic majority in Congress that is trying to help middle-class families.

The ads will be airing on broadcast and cable TV stations all week in the districts of Rep. Nick Lampson (D-Texas), who is one of the most vulnerable House Members of the cycle, and three other top GOP targets: Democratic Reps. Kirsten Gillibrand (N.Y.), Ciro Rodriguez (Texas) and Carol Shea-Porter (N.H.).

The ads, produced by the Democratic media firm GMMB, begin with a photo montage of high gas prices, home foreclosure signs, President Bush and other Republicans.

“While gas prices skyrocketed and Americans struggled, George Bush and the Republicans gave tax breaks to millionaires and oil companies,” a narrator says in the pro-Lampson ad. “But this month, middle-class families are the ones getting a break. Tax rebate checks are here. Because Congressman Nick Lampson and his colleagues in Congress know that in times like these, a little help can make a big difference for folks in Texas. And that’s the change we need.”

Jeremy Funk, a spokesman for Americans United, said the group has paid for “a low six-figure buy.”

Meanwhile, Sen. John Sununu (R-N.H.) is getting some help from a conservative third-party group running favorable advertisements in his state. The first-term Senator faces a challenging rematch against former Gov. Jeanne Shaheen (D) in November.

Citizens United on Friday launched a television advertisement, “Innovation,” hailing Sununu’s efforts to keep the Internet from taxation.

“Who could have guessed how the Internet would change our lives?” an announcer asks in the ad. “But while we dream of even more innovation, some politicians dream of more taxation. Fortunately, Senator John Sununu leads the battle in Washington to defeat Internet taxation schemes. Sununu helped deliver the 2007 Internet tax moratorium. And he’s pushing to keep the Internet tax free permanently.”

The ad will air for an undisclosed period of time. Shaheen’s campaign is already airing a television advertisement — a biographical spot reintroducing her to voters.