The legacy of Malcolm Turnbull's double dissolution looms over the coalition six years later, setting up the government with everything to lose in the Senate.
Nineteen of the upcoming 40 Senate seats are held by the Liberals and Nationals due to the way the election quotas were divided in 2016 when the entire upper house was dissolved.
Fifteen seats up for re-election are held by Labor and six by minor parties and independents.
Six Senate spots are up for grabs in each state, and the coalition - which runs on a joint ticket - holds three in every state except South Australia, meaning there is slim chance of picking up any further seats except for a third in the southern state.
Queensland's Senate race is one of the more unpredictable, with Labor and the LNP expected to hold two seats each.
Pauline Hanson is tipped to retain her spot, with a number of contenders vying for the sixth seat, including assistant attorney-general Amanda Stoker, United Australia Party leader Clive Palmer, former premier Campbell Newman and the Greens' Penny Allman-Payne.
The Nationals are set to pick up an extra seat from the Liberals in NSW due to former senator Fiona Nash being ruled ineligible to sit in parliament due to her British citizenship in 2017.
She was replaced by Liberal Jim Molan during a countback.
The Nationals' Ross Cadell will take second spot on the NSW ticket, with Liberal senator Concetta Fierravanti-Wells dumped after a factional fight with Senator Molan.
Long-time NSW Greens MP David Shoebridge will attempt to make a jump into federal politics.
Veteran Labor senators Kim Carr and the late Kimberley Kitching have been replaced on the Victorian ticket by Linda White and Jana Stewart.
SA is the only likely place the Liberals can offset any losses, with independent senators Stirling Griff and Rex Patrick both up for re-election.
While a resurgent Nick Xenophon is taking a second tilt at the Senate, it will be tough for him to reach a quota without a party and his name appearing below the line.
The Greens are pushing to pick up a seat in the ACT, plus Victoria, Western Australia and Tasmania, where they also need to defend incumbent senators.
Long-serving Liberal senator Eric Abetz is at risk of losing his seat after being relegated to third spot on the Tasmanian ticket.
Two Senate spots in both the ACT and Northern Territory are up for re-election, with the result unlikely to depart from one Labor and one coalition in each territory.
Former Country Liberal Party member Sam McMahon lost her NT seat in a preselection battle and will be succeeded by Alice Springs deputy mayor Jacinta Price.
A slate of independents - including former Wallaby David Pocock - are challenging Liberal minister Zed Seselja in the ACT, but a large enough swing to unseat the conservative is unlikely.
Whichever way the votes swing, the prospect of a major party forming a majority in the Senate is slim, with John Howard the last prime minister to command control of the upper house.
A majority is at least 39 out of 76 seats.
Currently, the coalition holds 36 seats, Labor 26, the Greens nine, and Pauline Hanson's One Nation two, with three independents.
