How credible is the Conservatives claim to add two million jobs if re-elected?

"Labour isn't working” was the Conservatives' powerful poster from the 1979 election campaign.

Unemployment then was just under six per cent, high by standards of the time, and the slogan tapped in to concerns about the economy after five years of a Labour government helping propel the Tories into power.

Fast-forward to today and unemployment is … just under six per cent after five years of a Conservative-led coalition government.

Yet far from weakness, economists are marvelling at the remarkable improvement in the labour market: around two million jobs have been created since the coalition was formed in the wake of the financial crisis, taking the total number of jobs to a new record.

• Read: David Cameron pledges two million new job

But the absolute number of jobs isn’t the key measure: waves of immigration mean there are a record number of workers in Britain as well and both figures continue to climb.

Instead, we should look at the proportion of people in work, which has indeed reached a new record of 73.3 per cent.

That leaves 1.86 million people who are officially unemployed. And it’s with an eye on this figure, perhaps, that the Conservatives’ latest election pledge is that they would create another two million jobs if re-elected. Does full employment beckon?

I’m not so sure. It’s true that the economy continues to recover and is expected to grow (and add jobs) in coming years. The number of vacancies is surging, especially at smaller firms, and in surveys bosses report that they intend to hire more people in the months to come.

The question is, can they get hold of the right workers to fill those jobs?

One feature of the recent surge in employment has been the return of older workers from periods of ‘inactivity’ combined with the decision by some to remain in work beyond when they might have expected to retire.

This pattern has stalled for much of the past year indicating one important source of workers is drying up.

The number of people who would work more hours if they could get them has fallen.

Pay is also beginning to rise as employers have to offer more to attract workers – who can afford to be more choosy.

Finally, the very fact vacancies are rising can also indicate that the employers are finding it hard to fill the roles. Britain may be running out of workers.

There are some other groups of workers which government policy could help back in to work.

The proportion of women in employment in the UK is 68.5 per cent - the highest since records began in 1971 but well below the rate for men (78.1 per cent) and below the comparable rate for women in Sweden (77.2 per cent).

To put it another way, there about two million of women not working because they are looking after the family or home.

Between 2009 and 2014 the economy added 400,000 jobs every year, on average (that figure masks the astonishing 617,000 added in 2014).

The UK would have to match that again if Tories were to fulfil their promise of another two million jobs by the end of the next parliament.

Economists I’ve spoken to today that this isn’t impossible but it would take an extraordinary effort to continue to create jobs at a rate they have already found mystifying.

But even if the jobs are there, Mr Cameron could discover that you just can’t get the staff.