Act leader David Seymour has launched an attack on National over its law and order policy and over a claim by National campaign chairman Steven Joyce that Seymour might be trailing in his Epsom seat.

He has compared National to criminals.

Seymour and Joyce are regular sparring partners in Parliament but it is usually friendly banter.

The tone of Seymour's attack on the law and order policy - to send the worst youth offenders to an Army-run boot camp - is more like that of New Zealand First or another Opposition party.

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"National are the worst re-offenders in youth crime", the headline of his press statement said.

He correctly pointed out that the boot camp policy was previously announced by John Key - in 2008 when he was in Opposition - and said it had failed because National did not want to commit to addressing youth crime beyond slogans.

"If National can't even break their own cycle of behaviour, how can they hope to break the cycle of failed families and youth offenders who cause so much damage to New Zealand society?"

Joyce made his comments about Epsom on TV1's Q+A programme in trying to downplay a poll in Ohariu which puts United Future leader Peter Dunne on 34 per cent beyond Labour's Greg O'Connor on 48 per cent for the electorate vote.

Joyce said that if there was poll in Epsom, "you may well find that David Seymour is behind in Epsom as well...but what we know is it tends to happen quite late, whether it be Ohariu or Epsom or anywhere else where these sorts of electorate contests are under way. I'm not too worried."

Seymour has seized on those comments because any suggestion that he won't win Epsom could suppress the Act Party vote nationwide.

He said he had commissioned a Curia poll in May which put himself 16 points ahead of Paul Goldsmith. (46 per cent to 30 per cent).

"In other words, the situation in Ohariu cannot be compared to that of Epsom."

"This is very important, because with confidence in Epsom, voters across the country will know that a party vote for Act is not wasted."

If a party wins an electorate seat, it does not have to reach the 5 per cent threshold of party votes nationwide in order to get more MPs from the list.

Both Act and United Future are support partners of the National-led Government and National campaigns only for the party vote in both those seats.