The poll showed 36% of 200 GPs who took part plan to vote Conservative at the next general election - down from 48% at the 2011 general election.
Support for the Liberal Democrats also fell sharply.
Dissatisfaction with the coalition government has resulted in a big swing to Labour, with 25% of GPs now backing the party, up from 13% in 2011.
GPs are unhappy with government policies, with many citing the NHS reforms as a factor in their decision to vote differently at the next general election. But many feel none of the main parties can now be trusted with the NHS.
One said: 'I now believe all three main parties cannot be trusted to protect the service.'
Another said: 'I no longer trust Labour to have the working person's interests at heart; I have never trusted the Tories. The only remotely left-of-far-right option seemed to be the Liberal Democrats last time and I have no idea who is left now.'
More GPs (66%) rated workload in the top three concerns for the profession in 2012. A total of 55% rated practice funding in the top three, followed by pension cuts (52%) and commissioning (50%).
One GP said: 'The workload is unsustainable and I keep looking for an exit strategy. Helping patients sustains me but most of the time it is sheer misery.'
Another said: 'Practice income is decreasing while expenses are rising rapidly. If this goes on we will need to consider which services to cut.'
GPC chairman Dr Laurence Buckman warned last week that pressure on funding may force GPs to make staff redundant this year (GP, 11 January).
GPC deputy chairman Dr Richard Vautrey said it was 'no surprise' support for the government was falling. 'GPs feel under attack on pensions particularly and the Health Bill is another major concern.'