Bob Hawke will urge Australians to consider Bill Shorten's union background as proof he can unite the country under a Labor government, fending off attacks on the Opposition Leader in the frantic final days of the election campaign.

The former prime minister will issue an open letter to all voters on Wednesday to counter Coalition attempts to "divide and frighten" voters and argue instead that Mr Shorten had shown the "political courage" needed to lead the nation.

Bob Hawke says Bill Shorten has set out a large policy agenda rather than being a "small target" politician. Credit:Labor staff photographer

Prime Minister Scott Morrison has ramped up his accusations that Mr Shorten will succumb to "militant unions" if Labor wins the election this Saturday, following years of attacks on his past as leader of the Australian Workers Union.

Six days after signing a joint letter with former prime minister Paul Keating to endorse Mr Shorten's economic agenda, Mr Hawke will enter the campaign again to praise the Opposition Leader for setting out a large policy agenda rather than being a "small target" politician.