Some of the loudest and least dignified battles of the 2015 election will take place in the seats whose results will turn on a few hundred votes.

These are the ultra-marginals: constituencies that could change hands on the tiniest of swings, and where Conservative and Labour candidates will need to fight street by street to win a majority.

They are the crucible of the next government, for if the Tories hold all the seats they won in 2010 and gain another two dozen or so more ultra-marginals, they will win the election and seek to govern by themselves.

But of the Conservatives’ existing seats, no fewer than 14 are ultra-marginals themselves: constituencies where the sitting MP has a majority of under 1,000. If the party cannot hold on to all of these, it follows they will need to make even more gains elsewhere, and so the quest to become the government becomes that much harder.

Another survey is due this week from Lord Ashcroft, carried out in these very seats: the Tory-Labour marginals. We should see polls in constituencies with some of the smallest majorities in the land, as well as seats at the very top of the Conservative and Labour target lists.

It follows Lord Ashcroft’s recent snapshots of Lib Dem-Tory and Lib Dem-Labour marginals, and comes a few months after he commissioned his first survey of Tory-Labour battlegrounds. On that occasion, he found an average 6.5% swing from Conservatives to Labour: enough to defeat around 80 Tory MPs and give Labour a majority in parliament.

Ashcroft has already tweeted that his new survey contains a few “surprises”. I’m guessing it might turn out that the average swing this time is smaller, or more inconsistent.

As I’ve done ahead of each of Ashcroft’s recent batches of polls, here’s a preview of some of the seats likely to feature.

First, the Tory seats most vulnerable to Labour:

All of these would fall on a swing of just under 1%.

It’s worth remembering just how many ultra-marginal seats the Tories are defending. Look at the majority of the 10th seat on the list, Wolverhampton South West: just 691. The 10th most vulnerable seat on the equivalent list of Tory-Lib Dem marginals has a majority of 2,691 (Weston-super-Mare).

I’ve noted before just how many of these seats are currently held by MPs only elected in 2010, and who have therefore no experience of having to fight for re-election.

By contrast, here are the Labour seats most vulnerable to the Tories:

These would fall to the Tories on a swing of around 1.5%.

Labour doesn’t have quite so many ultra-marginals on its hands as the Conservatives, though they also have two with majorities under 100 (and coincidentally both parties have a constituency where their majority is 92).

The 10th seat on this list belongs to Ed Balls, the shadow chancellor. The equivalent seat on the list of Labour-Lib Dem marginals has a majority of 1,724 (Edinburgh North & Leith).

If Lord Ashcroft’s survey reveals that even one of these seats may fall to the Tories next year, it would send the Labour front bench off on their summer holidays underneath one almighty cloud.

But as I’ve said before, when it comes to the question of whether Labour might lose seats at the election, the answer can only be: of course they could.