NSW Nationals MPs will be anxiously eyeing off the results in this weekend's by-elections, as they try to read the tea leaves for their own electoral fate in the 2019 state election.

Elections today held in: Blacktown electorate includes the Sydney suburbs of Marayong, Kings Park, Doonside, Woodcroft and parts of Blacktown, Quakers Hill, Lalor Park and Seven Hills

Blacktown electorate includes the Sydney suburbs of Marayong, Kings Park, Doonside, Woodcroft and parts of Blacktown, Quakers Hill, Lalor Park and Seven Hills Cootamundra electorate includes the major centres of Cowra, Grenfell, Young, Cootamundra, Gundagai, Junee, Narrandera, Temora and West Wyalong

Cootamundra electorate includes the major centres of Cowra, Grenfell, Young, Cootamundra, Gundagai, Junee, Narrandera, Temora and West Wyalong Murray electorate includes the centres of Deniliquin, Leeton, Griffith, Hay, Hillston, Balranald, Moama and Wentworth

Labor is expected to easily hold its safe seat of Blacktown, but the Nationals are under threat from the Shooters, Fishers and Farmers Party in the previously safe seat of Murray and are also bracing for a voter backlash in Cootamundra.

When the Nationals lost the Orange by-election last year the shockwaves reverberated through the Government, swiftly forcing Nationals leader Troy Grant to fall on his sword and politically wounding then-premier Mike Baird.

While a potential loss in Murray this weekend is not expected to spark such a violent and immediate political reaction, it would signal there are much deeper problems for the Government as it tries to pave its way to a victory in the 2019 state election.

On one hand Orange was a perfect storm for the Shooters, Fishers and Farmers Party, with voters punishing the Government for trying to ban greyhound racing and forcing council mergers in the bush.

But a loss or mammoth swing against the party in either Murray or Cootamundra this weekend would indicate there is a broader and more lasting problem with the Nationals brand.

It signals that the policy reversals on greyhounds and council mergers are not enough to convince voters that the Nationals will stand up for the bush — and the party only has 17 months until the next state election to change this perception.

Nationals asking voters for more time

This week, Nationals leader John Barilaro all but begged voters for another chance, promising "if they keep us in this time and we don't deliver by 2019 then they have the opportunity to throw us out then".

He tried to blame the ill will against his party in the electorates on problems that his Government had "inherited from Labor", citing issues such as lingering anger over the establishment of the River Red Gum National Park.

But those issues did not prevent retiring Nationals member Adrian Piccoli from winning the seat with a whopping margin just over 18 months ago — so it is very difficult to buy that explanation from Mr Barilaro.

While by-elections usually see much bigger swings than general elections, if seats with such large margins can be at risk this weekend then Nationals MPs sitting on much smaller margins will be fretting about an electoral bloodbath in 2019.

Mr Barilaro will be keenly aware of that issue — he holds his seat of Monaro on a margin of only about 2 per cent.

History has shown that nervous MPs are poisonous to the prospects of a unified government and strong leadership.

And Labor would be rubbing its hands, because if Murray is picked off by the Shooters this weekend then there would be one fewer seats that it would have to win to snatch government in 2019.