Inside the debate halls, the clash may be Republican versus Republican. But offstage, conservatives are mounting a unified and expensive air assault on the candidates’ common opponent: President Obama.

Nearly a year before Election Day, Republican presidential candidates and conservative action groups are already spending heavily on television advertising aimed at casting Mr. Obama as a failure.

Their tactics, the aggressive and sometimes misleading kind not typically used until much further along in a campaign season, have led to a spat with Democrats in what is shaping up to be the most costly election advertising war yet.

In an advertisement from Gov. Rick Perry of Texas that is now running on national cable television, Mr. Perry looks directly into the camera and declares: “Obama’s socialist policies are bankrupting America. We must stop him now.”