Labour leader Ed Miliband may have dominated the annual party conference season with his surprise energy freeze pledge, but he has done that before. The real question for the Opposition is whether he can sustain his advantage.
The challenge for the prime minister is to show just how he intends to respond to Labour's new, decidedly left-of-centre stance and offer something as compelling.
The first meeting on this new, post-conference battlefield came during the first Prime Minister's Questions of the parliamentary year.
Miliband was on the offensive and used the bulk of his allocated time demanding to know how the government would match his proposal, and showing he is determined not to let this one slip away from him.
As for the prime minister, he gave the clearest possible signal that he will be running a "Red scare" election campaign, with talk of Miliband's "Marxism".
He didn't mention the Daily Mail's controversial profile of Miliband's Marxist father, Ralph, but he didn't have to.
What he did say was: "I know he would like to live in some sort of Marxist Universe where you can control these things (energy prices) but he needs a lesson in economics."
The suggestion was there for anyone who wanted to hear it, that the son is heir to the father's Marxism.
Miliband's response was to laugh. Not even his most optimistic left-wing supporters believe he is a Marxist, or even an old-style, command economy socialist.
But this is a strong line of attack from Cameron who is tapping into memories of past Labour governments who have been more reasonably accused of toying with socialism. He even revived the 1992 election "Double Whammy" slogan to describe Labour's tax and spend proposals.
The trouble for Cameron was that he seemed unsure whether to pursue this line or actually suggest he would be even more effective at lowering prices than Labour.
Perhaps surprisingly, Miliband did not mention the government's morning announcement that it would impose a cap on rail ticket prices, let alone its intervention in the housing market with mortgage guarantees.
He did try to pre-empt the expected attacks on his "socialism" by pointing out that Chancellor George Osborne had described the energy freeze as something out of Das Kapital while Cameron seemed to think there was something in it.
"Is freezing energy prices a good idea or a Communist plot," he asked.
"I'll leave the Communist plots to you," was Cameron's reply.
But Miliband had hit a nail. The government appears unsure how best to respond to him. Is he a raving Marxist or Communist or are his plans, as Cameron also said, just unworkable gimmicks.
This first clash of the season showed that Miliband, at least, has decided in which direction he is going to drive his election campaign while David Cameron still has to make his mind up.
With the unofficial campaign already under way it is an indecision the prime minister will not be able to maintain for long.