Having your leadership locked into a single issue is never a good idea.
John Key did it, promised that the age of the pension would never rise above 65 while he was the Prime Minister. He had his Government stick to it even though the pressure was growing to raise the age.
That all changed when the pragmatic Bill English increased the age on taking over the Beehive's ninth floor office, but hedging his bets with the electorate by having the higher age starting to kick in when they're all in retirement homes.
Now Jacinda Ardern's painted herself into a corner, and like Key, it's not an insignificant economic one.
Simon Bridges was on fire, clearly relieved not to be talking about himself and how he's been absolutely banging on about everything concerning the way he's handled, and deflected the hand grenades from his rogue, former MP Jami-Lee Ross.
He was back on his bandwagon, that's becoming daily more expensive to run, thanks to an uncaring, promise-breaking Government.
Bridges was up off the mat in Parliament's bear pit trying to put Jacinda Ardern on it by asking her about their regional fuel tax, and why councils around the country will be able to make bids to use it from 2021.
Rising fuel prices touch a raw nerve in the Beehive, for a myriad of reasons, not the least because it breaks the no-new-tax promise and disadvantages the very people they say they are there to protect.
Ardern agreed there was a date from which bids could be made, but she gave the grizzlies an assurance that while she's the Prime Minister, which at the moment she says feels as though it could be for quite a while, there'll be no new fuel taxes in regions other than Auckland where it was imposed in July but felt as though it was being imposed on all of us given that it appeared to increase the price nationwide.
So was this another rush of blood to the head, like the ban on oil and gas exploration? Certainly, it seemed to come out of left field.
Appropriately standing in for the Transport Minister, the self-proclaimed first citizen of the provinces, Shane Jones, was asked when he knew about Ardern's willingness to put her job on the line to protect the regions' motorists.
Jones confirmed it was a promise on the hoof, answering: "When the Prime Minister stood up".
She will now have to stand up to her Finance Minister, to explain where the money's going to come from for the provinces' badly needed spend on transport infrastructure.