“Your husband came third in the 2010 Labour leadership election,” a reporter observed at a lobby lunch for Yvette Cooper. “Has he given you any tips on how to avoid ending up in the same place?” Cooper gave only the faintest flash of annoyance, a masterclass in self-restraint considering this was at least the fifth time within an hour that Ed Balls had been namechecked. However hard the shadow home secretary tries to step out of her husband’s shadow, people are queueing up to push her back into it. Radio 4’s Woman’s Hour was first on the bandwagon. Vive la Sisterhood.

To be fair, it had been easy to mistake Cooper’s apparent dullness for deference in the last parliament. Whenever she and Balls were in the same room, it was he who always seemed to get the first, second, third and, well … every word in; not all of them as well thought through as they might have been. Guilt by association became the accepted wisdom. Since Balls lost his seat, though, Cooper has become a different woman in Westminster. Confident, well-informed and engaging. She now treats her husband in public as she probably always did in private. With long-suffering forbearance.

“The one thing Ed has told me,” she said, “was that by the end of the hustings every contender will have stolen each others’ stories.” It was an astute observation. With only one down and at least 15 more to go, it’s happened already. On Wednesday night in Nuneaton, Liz Kendall scored a point over Andy Burnham by saying “country first”. This has now also become firmly incorporated into the Cooper narrative. “Country first,” she insisted, “though obviously the party is very important, too.”

At least we turned down the fiscal policy water feature at the seventh planning meeting Yvette Cooper on the Ed Stone

Come September it may well be hard to tell the three main contenders for the leadership – Cooper, Burnham and Kendall – apart. For all the rhetoric, all three are for much the same thing. “Learning from our mistakes. Reaching out to all parts of the country. Moving forwards while looking backwards and not being afraid to step sideways.” What will really make the difference is how much they can convince members of the Labour party that they have the requisite personality, stature and quick-wittedness to lead the party and go head-to-head with David Cameron. Cooper has always been written up as the safe pair of hands: the person who could be relied on to not make a bad situation any worse. On this showing she might well prove to be rather better than that.

Disguising her disappointment that the lobby lunch was less formal than she had been led to believe, Cooper remarked: “I was expecting a packed lobby, not a packed lunch.” OK, so this wasn’t the funniest gag ever, nor had she written it herself, but the audience was still wrongfooted. This was the first time anyone had heard Cooper say anything that was meant to be funny at which anyone had laughed.

This, though, was just the start. Having sounded like someone you wouldn’t mind sitting next to at dinner as she whizzed through her spiel on Labour and Europe, she had enough confidence to save her best joke for questions. When someone mentioned the Ed Stone, she said: “At least we turned down the fiscal policy water feature at the seventh planning meeting.” This was all the funnier for probably being true.

It’s also hard not to imagine that Cooper is the person Cameron would least like to face at PMQs. He’s at his worst with women; especially those who are brighter than him. What’s more, she should be more than able to deal with her past. “This is the woman who is married to the man who wrecked the economy,” the prime minister may say in October. To which she will reply: “This is the man married to a woman who works for a tax haven-based luxury goods company and whose father-in-law makes £350,000 per year from a publicly subsidised wind farm on his estate.” That should just about do it.