Rarely is an opposition party gifted such a golden opportunity to really put the boot into an old foe as National is currently offered by Winston Peters' troubles over political donations.
But National has pulled its punches, prompting accusations in some quarters that it is being too soft on him because it fears alienating the NZ First leader this side of the election when it may well need him to help form a government afterwards.
That constraint has nearly had John Key tied up in knots as he calls on the Prime Minister to set up some kind of inquiry to investigate whether her Minister of Foreign Affairs has complied with ministerial guidelines and other rules covering donations, only in the next breath to refuse to rule out the possibility of National working with Peters.
The reticence has been reflected in Parliament with National focusing its attack on the Prime Minister partly out of necessity - Peters is overseas on foreign affairs business - and partly to undermine Helen Clark by arguing she is protecting Peters simply to ensure Labour can cling on to power.
After two days of questioning in the House, National's leader has got little change out of Clark who has instead blunted National's line of attack through constant reference to what might be called National's "glasshouse syndrome" - as in those who throw stones shouldn't when their party has a history of happily accepting anonymous and corporate donations.
It didn't help Key that his colleague Nick Smith made the mistake of responding to Clark's taunts about the "Freedom of Speech" Trust set up to help pay Smith's legal bills. Smith replied that he had sought advice from the registrar of pecuniary interest as to what disclosure requirements he should follow. He was told he only needed to declare a pecuniary interest in a trust.
Quick as a flash, Helen Clark retorted that if Smith did not have to declare either debts or gifts, "then nor does Mr Peters".
The big question is what National would have done had Peters been present.
National has reasons to be cautious about confronting him head-on. Given relations between NZ First and National are still pretty brittle, vituperative attacks on Peters have no long-term benefit, even if they would give short-term satisfaction to those National MPs who hate him.
Such attacks might actually help Peters by engendering a backlash from voters willing to be persuaded that Peters is the victim of some media-driven smear campaign.
It makes more sense for National to highlight the Prime Minister's predicament and put pressure on her to at least stand Peters down from his ministerial roles against his wishes, thus causing friction between Labour and NZ First.
However, the murmurings that National is being too soft on Peters may have prompted the party's third-ranked MP, Gerry Brownlee, to offer some harsher criticism of the NZ First leader during yesterday's general debate.
Brownlee accused Peters of touting himself as "one of the great parliamentarians of our age" when his failure to disclose the $100,000 donation from wealthy expatriate Owen Glenn actually displayed Peters' "utter and complete contempt" for Parliament.
Peters' claims about what he knew or rather did not know about the donation were "unbelievable", "irresponsible" and were "destroying the credibility of Parliament" as an institution.
Strong language it may have been. But it turned out to be only a fleeting departure from National's script which saw following National speakers in the debate focus the attack firmly back on the Prime Minister.