When the earth's plates move, earthquakes and volcanoes follow. This seems to be happening in the wake of the decision to hold, 50 years after their invention in the United States, debates between party leaders.

The Liberal Democrats, the Cinderella of British politics, were always keen, as have been the media, but prime ministers have taken the view that debates other than those in the House of Commons are unseemly, and party leaders holding a lead in the polls haven't seen an advantage. This time was different.

Elections are determined by remarkably few voters. These are those voters who choose to vote (in the past two elections only six in 10); who have little party loyalty (about one in five); and who live in marginal seats. As few as one voter in 25 decides who rules the land.

Reuters has commissioned Ipsos MORI to carry out surveys in "Battleground Marginals", defined as seats the Conservatives must win to have an overall majority, requiring roughly a 6% swing from the 2005 election. For a solid working majority, they need as much as a 9% swing.

In these seats in 2005 the vote averaged 45% for Labour, 31% for the Conservatives and 17% for the Liberal Democrats (who won none of them). Before the election was called, there had been a 5% swing to the Conservatives: good, but not good enough. Using the uniform national swing model, which with limitations is a good starting point, such a swing would leave the Tories a sandwich short of a picnic.

With the baseline survey pre-election, and another during its first week or two with little change, Labour was on 41%, down four, the Tories up six to 37% and the Liberal Democrats on 11%, down six, with the other minor parties up from 7% to 12%. Then came the debate.

Among the electorate as a whole in these key seats, there was a 12% swing from the other parties to Nick Clegg's Liberal Democrats. More than half (52%) of the votes came from Labour, a third (37%) from the Tories, and 11% from the others. This dropped the Tory swing down to 4% – in terms of seats, a long way from their long-sought majority.

So where did these new Liberal Democrat votes come from? In these battleground seats, quite a lot more from women (+16%) than men (+9%), more from middle-class voters (+15%) than working-class (+9%), and more from the young, 18-34s (+16%), than middle-aged, 35-54 (+13%), or from older people, 55+ (+9%).

The retention rate is also a factor in the rise of the Lib Dems. From the data before the prime minister called the election and after, the Tory retention rate fell eight points, from 77% of those who recalled voting Conservative in 2005; Labour held its own and a bit more, from 52% before, and 55% this past week. The Liberal Democrats, however, bounced from a lowly 43% up 18 points to 61%.

This volcanic activity seems to be persisting; voters and now bookies signal a hung Parliament. That means Clegg, while not becoming politically king, could certainly become a kingmaker.