Centre Alliance - formerly known as the Nick Xenophon Team - will block franking credit changes worth $5 billion a year unless they are grandfathered, and propose a cap on the number of properties that can be negatively geared, rather than abolishing the tax break for existing homes.

If Labor is elected, Bill Shorten is likely to face Senate crossbench opposition to signature policies.

In a Senate race that will define Labor's ability to govern effectively if it wins the May 18 election, analysts believe the South Australian party is likely to have the balance of power in the 46th Parliament.

The party looks set to make up the core of negotiations on the crossbench, unless Labor and the Greens can muster a combined 39 seats - a prospect analysts at the left-leaning Australia Institute have conceded is unlikely.

Election analyst Kevin Bonham said One Nation was almost certain to regain the seat it lost through the defection of Fraser Anning in Queensland, with former senator Malcolm Roberts likely to return. 

Legislation needs a majority of 39 votes to pass the Senate. Labor and the Greens currently have 35 seats combined.

"Based on national polling the likely prospect is about two gains and they fall just short of a combined majority," Mr Bonham said.

"The first cab off the rank will be Centre Alliance. There is a realistic prospect that you have Labor-Greens-Centre Alliance combined majority."

Key Centre Alliance crossbench senator Rex Patrick will hold his seat until 2022, alongside Stirling Griff. The party intends to launch its election campaign in Adelaide on Friday.

Senator Patrick said the party would target Labor over its changes to franking credits.

"It is not proper to make changes once people have retired and set themselves up in a particular position," he said.

SMH/The Age