One of the most persuasive trends in British politics over 2014 has been the decline in support for the two main parties.

The latest opinion polls show the combined vote share for Labour and the Conservatives to be averaging 64%. The implication is that over one third of the country backs neither of the UK’s largest parties.

That figure has been dropping throughout the past 12 months. At the start of 2014 the combined share for Labour and the Tories was 70%. Since then there has been a fall of six percentage points.

What’s more, the current figure of 64% splits 31.2% for the Tories and 32.8% for Labour. Neither party is attracting even a third of the total vote.

I’ve looked back at all the polls not just in 2014 but 2013 as well. The trend is evident before this year, though not as pronounced:

The two parties began 2013 on a combined share of 73.2%. By the end of 2013 it was down to 71.7%, but had dipped as low as 68.2% in June (a consequence of a spike in support for Ukip after the May 2013 local elections).

If I shrink the y axis of the graph to show just a 60-75% range, you can see more clearly how the trend accelerated after 2013, particularly from August 2014 onwards:

The acceleration has happened because support is now fragmenting in more directions.

Up until early 2014, it was really only Ukip that was attracting voters in significant numbers from Labour and the Tories. Since then, first the Greens and then the SNP have joined the process, both winning support that is large enough in size to show up in opinion polls.

But while the Tories are polling roughly the same now as they were two years ago (their average for January 2013 was 31.7%; their average for this month is currently 31.2%), Labour’s share has slumped. The party averaged 41.5% in January 2013; its average for this month is currently 32.8%.

It is therefore Labour who is most to blame for the net decline in the combined vote share for the two main parties. There has undoubtedly been a lot of churn within the figures – for example, Tory support drifting to and from Ukip – but the direction of the overall trend must be ascribed chiefly to the unpopularity of Labour.

One of the biggest questions for 2015 will be whether this trend continues, or if a floor has been reached and the combined share for Labour and the Tories has fallen as far as it can go.

My instinct is that we’re nearing the floor. I don’t think either of the main parties will sink below 30% permanently. If anything, there should be a slight uplift in the figures over the next few months, as voters return to Labour and (particularly) the Tories ahead of polling day. But I’m not sure the combined figure will climb above 66%. I suspect one third of the country will remain put off the two biggest parties for a good few years to come.