The crucial role the “super PAC” now plays in modern presidential politics has been on vivid display in the week before the Super Tuesday primaries, as these outside groups have all outspent the campaigns and become their de facto advertising arms.

The super PACs supporting Mitt Romney, Rick Santorum and Newt Gingrich have poured nearly $4 million into advertising in Ohio ahead of the primary next week, accounting for most of the spending on commercials there in what has become an overwhelmingly negative contest.

Beyond Ohio the story is the same. The money spent by super PACs, another $8 million, continues to outpace what candidates themselves are willing and able to spend. Mr. Romney, whose campaign spent almost three times as much as it brought in during January, has chosen not to advertise in any Super Tuesday state but Ohio. He has committed about $1.2 million to advertising there, according to figures provided by media strategists.

Meanwhile, Restore Our Future, the super PAC supporting Mr. Romney, has sunk about $7 million into advertising on broadcast television, cable and radio in the last month, blanketing the airwaves from Idaho to Georgia.