Canvassing for Votes, one of a series of four wonderful paintings by William Hogarth about the corruption of parliamentary elections in the 18th century, depicts agents for the Tories and the Whigs flourishing banknotes at an innkeeper in an attempt to bribe him. Would never happen today, of course. Payment in cash or kind in exchange for a vote – the practice that used to be called “treating”– is strictly illegal. Anyone caught doing it will likely wind up in jail.

That’s an obstacle for vote-hunting politicians at election time. Fortunately for them, the law has a loophole. And that loophole is massive. There is nothing on the statute book that says a politician can’t offer a bribe so long as it is directed at lots of voters. Individual bribery is a crime; mass bribery is entirely legal. Which is a good thing for David Cameron and Ed Miliband. If mass bribery were not allowed, both would be facing prosecution for the promises they have been making in the past few days.

The Tory leader has been digging into his pocket – or rather digging into other people’s pockets – and waving wads of cash at older voters. “Roll up, you wrinklies! Free stuff here!” I paraphrase Mr Cameron, but only by a little bit. For as long as he is prime minister, he has promised, all pensioners, even the most affluent, will retain all their welfare perks. The free TV licences, the free bus passes, the winter fuel allowance, whether pauper or billionaire, the lucky pensioner will keep the lot. He has also pledged to continue with the “triple lock”, which guarantees increases in the state pension regardless of what is happening to the economy or how other segments of the population are faring. This is his latest instalment in a series of cash offers to those at the upper end of the age scale.

The least subtle of the Tory inducements is an extension to the “silver bonds”, which give a preferential rate of interest on savings – a subsidy to more affluent pensioners, paid for by the government from the money of other taxpayers. That is so close to being a straightforward cash bribe to vote Tory that it might have been more efficient for the Treasury simply to stuff the money in brown envelopes and mail it out with Conservative campaign literature.

One consequence of the goodies promised to older voters is that all the further welfare cuts planned by the Tories will fall on the working-age population. There are even Tories who privately think this can’t be right, intellectually, economically and in terms of inter-generational fairness. There are quite a lot of wealthier pensioners who question whether they should be treated so generously compared with other age groups.

The prime minister attempts to make his naked bribes look a bit more respectable by clothing them in warm words about the older age group. “These people have fought wars, seen us through recessions, made this the great country it is today.” The truth is that this has nothing to do with reverence for wartime sacrifice. An ever-increasing number of the beneficiaries were born too late to have been fighting the Nazis. As for recessions, it is people of working age, especially the young, who have been at the sharp end of austerity.

We all know why the Conservatives are flinging bungs in this direction. There are more than 12 million pensioners. They are more likely to vote than any other age group. The more affluent elderly are very likely to vote. The Tories are hoping to gain an advantage in the competition for silver votes with the Liberal Democrats, Labour and Ukip. Such a close election will be decided by who votes. Differential turn-out will be key. The more of your potential electorate that you can get out, the more likely it is that you will find yourself in Number 10 on 8 May.

This is also the main reason why Ed Miliband is treating for votes at the other end of the age spectrum by promising to cut student tuition fees to £6,000. One way of thinking about the Labour leader’s pledge is to ask what problem this is supposed to fix. Since tuition fees were raised to £9,000, applications for university places have not gone down; they have gone up. Applications from students with less advantaged backgrounds have not gone down; they have also gone up. No one starts repaying the loan until they are earning more than £21,000 a year and any debt outstanding after 30 years is written off. Of course, when I talk to students I hear complaints about them being burdened with debt. But it is often a greater source of irritation to them that they don’t think the teaching they are getting is value for the money. Some of the sharpest complaints about that are from students at universities with the more prestigious names. That might have been a useful area for Labour’s attentions. The upshot of Mr Miliband’s policy is that the greatest beneficiaries will be the highest earning graduates. Who knew that the Labour leader came into politics to redistribute money to future bankers?

His promise will cost approaching £3bn a year. Senior colleagues don’t much disguise their feeling that there are better ways to spend that sort of money. On more quality apprenticeships for the half of the young who don’t go to university, say, or on enhanced early years intervention. Internal resistance to Mr Miliband accounts for the tortuous manner in which Labour has produced a pledge which he first floated in the autumn of 2011 in an interview with The Observer. From conception to birth, it has taken more than three years. The woolly mammoth had a shorter pregnancy.

Just as Mr Cameron dresses up his kickbacks to pensioners with words about supporting dignity in old age, so Mr Miliband tries to make his cash offer to students sound less like a bribe by clothing it in rhetoric about fulfilling “our responsibilities to the young”. The truth is that he is also driven by the hunt for votes. The tuition fee cut was tested by Labour on focus groups of younger people. I’m told that it was wildly popular. In terms of voting intention, Labour has a substantial lead among younger voters. A chunk of the constituencies that Labour needs to win in order to be the next government just happen to be university seats with large populations of student voters. Labour’s big worry is that they won’t turn up at polling stations on the day. It also wants to stop younger voters drifting over to the Greens. The tuition fee cut will be, so Labour strategists hope, a major inducement for student voters to make the journey to the ballot box and give their cross to Labour. It is also a way of locking in former Lib Dem voters by reminding them of Nick Clegg’s pre-election bribe last time around – the one that boomeranged so disastrously for his party when they unexpectedly found themselves in government and didn’t deliver.

I’m not pious about this. To be shocked that politicians try to buy votes at election time is to be astonished to learn that bears defecate in the woods. When an election is as tight and uncertain as this one, there’s even more likelihood that the parties will try to bribe people with their own money or other people’s money.

What does bother me is the risks they are taking in making politics so explicitly and crudely transactional. The overall impact could be to make an already disenchanted electorate even more cynical and sour.

It is confusing the messages that the parties are sending to the country as a whole. One Tory election slogan is: “Britain Living Within Its Means”. Stick with us, they say, because the job of paying down the deficit is only half done. At the same time, they are claiming to have loadsamoney to bung in the direction of favoured groups of voters. Their biggest – and totally uncosted – election bribe is more than £7bn of tax cuts. Which is the Tory pitch? Stick to a hard but worthwhile road? Or, money grows on trees? Tories privately express anxiety that their message has become contradictory and confused.

As for Labour, it has been trying to burnish its fiscal credentials by saying it won’t make lavish promises and acknowledging that it would have to make spending cuts, if less severe ones than those planned by the Tories. “Big reform, not big spending” has been one of Mr Miliband’s slogans. I see the big spending in his promise to cut tuition fees; I don’t see much by way of big reform.

There’s something dispiriting, narrowing and lowering about parties so obviously targeting selected segments of the population rather than trying to broadly appeal across the nation. Our polity is already fractious and fragmenting. Brandishing cash sweeteners so squarely directed at different age groups opens another fracture along generational lines. Those voters who aren’t being offered bribes are likely to feel resentful. Even those favoured groups who are having banknotes waved at them may end up feeling cheapened by this descent into cash-and-carry politics.