(Bloomberg News) Watching the other Republican candidates struggle to combat the appeal of Herman Cain's 9-9-9 tax plan, I am reminded of a Senate campaign that I worked on in the 1980s, when our candidate was outspoken on a single issue.

Our pollster came in to brief us and said, "Sixty percent of voters in this state identify our candidate with this one, single issue."

Horrified, a colleague sputtered, "How can we win if the voters think of our candidate as a single-issue candidate?"

The pollster smiled: "You don't understand. Yes, in the voters' minds, our candidate is connected with only one issue, and isn't yet at two -- but all the others in the field have zero issues, and are struggling to get to one."

Likewise, in this year's Republican contest, Cain's momentum comes both from the 9-9-9 plan itself, and from his focus on at least one clear idea -- misguided as it may be -- in a field of candidates who have none.

This is a wake-up call to Cain's rivals that they won't be able to stumble through the primaries merely by bashing President Barack Obama and compiling longer and longer lists of administration initiatives to roll back: They are going to have to put forward affirmative proposals.

A more immediate challenge for Cain's opponents, however, is deciding how they will deal with the 9-9-9 plan, now that his candidacy and his proposal have taken center stage.

Cain's Surge

Their first approach -- just ignoring the plan -- was an utter failure; each poll over the past month has shown Cain rising in popularity while his opponents failed to take him or 9-9-9 seriously.

Their second approach -- slapping at the proposal with dismissive jabs (such as Michele Bachmann's comment that voters should "turn 9-9-9 upside down" and appreciate that "the devil is in the details;" or Jon Huntsman's jibe that he thought "9-9-9 was a pizza price") -- likewise was unsuccessful. Cain had his very best debate night at Dartmouth College last week, and the half-hearted swipes only served to elevate the visibility of his proposal, not take it down.