The party emerging as Senate kingmakers will punch a multi-billion dollar hole in a Labor government budget, after declaring its opposition to raising the top income tax rate and two of Labor's signature policies.

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. Credit:Dominic Lorrimer

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.