Labor would likely enjoy a more favourable Senate if it wins government in May but some key planks of the party's policy agenda could nonetheless be held hostage to a still-powerful crossbench.

According to projections of the Senate's composition after the election, Labor and the Greens could together hold 36 to 38 seats, just shy of the 39-seat majority needed to pass legislation through the upper house.

The Senate could pose difficulties for a new Labor government. Credit:Alex Ellinghausen

The analysis predicts that, even if a Labor government enjoys Greens support on legislation, the party would still need to secure the votes of Centre Alliance, One Nation or other crossbenchers.

A survey of those crossbenchers' positions suggests mixed fortunes for Labor's policy agenda, with success almost assured for some measures but others facing an uncertain future.