The party leaders are reluctant to talk about what they might do if none of them wins a clear majority on Thursday. But, as keen students of political history, they know the half-forgotten secrets of coalitions, minority governments, pacts and haggling over electoral reform that may come into play on Friday.

Such dilemmas were common in the 19th century, but 20th-century Britain faced them at least once a generation. After the 1923 general election the party with the most seats did not form the new government. In 1930, parliament came close to introducing the very voting reform which Gordon Brown has just offered for a referendum 80 years later.

The best-remembered decade of hung parliaments and backroom deals is the 70s. In February 1974, the defeated Conservative PM, Edward Heath, tried to lure the then-Liberals into a coalition. He failed, but three years later, Labour's prime minister, Jim Callaghan, negotiated an informal understanding, known as the Lib-Lab pact, with David Steel. It helped keep his minority party in power for two years.

Whatever they say before polling day, both David Cameron and Gordon Brown may try such tactics again this time. On both occasions before, Liberal third-party demands for electoral reform proved crucial to the haggling.

Cameron's insistence that he will seek to govern alone even without a majority suggests the topical precedent may be 1923 – after the belated overthrow of David Lloyd George's rackety Lib-Con 1918-22 coalition by Tory backbenchers. Then, the new Tory PM, Stanley Baldwin, called a snap election over free trade, only to lose 86 seats – from 344 seats to 258. Still the largest party, the Tories tried to battle on with the aggressive minority-government tactics which Alex Salmond's SNP administration currently deploys in Holyrood.

In effect, the incumbent leader dares his opponents to bring him down and risk another election, which he wins. Both Cameron and Brown have been urged to adopt this tactic if they emerge with most seats on Thursday. So was Heath in 1974.

Alex Salmond has survived since 2007 in this precarious fashion. But Baldwin's bluff was called. His 1923 King's Speech programme voted down by Labour (191 seats) and the resurgent Liberals (158), enjoying their last big push, one which not even Nick Clegg can hope to match.

In May 1929, MacDonald's hopes were quickly derailed by the Wall St crash on 24 October. The Liberals had the best recovery plan, but MacDonald mistrusted their leader, the brilliant Lloyd George. Only when weakened by the resignation of Sir Oswald Mosley, the future fascist leader, did he allow talks on electoral reform to go forward.

The situation is not unlike Labour's today. Hit by an economic crisis which could have become as bad as the Great Depression, a weakened Brown is now offering a referendum on the alternative vote (AV). Not strictly proportional but grudgingly accepted by Nick Clegg as potentially a small step forward, it gives voters a second vote, requiring a winning candidate to get more than 50%.

In 1929, as now, Labour was divided. Arthur Henderson, the foreign secretary, was keener on AV than his PM, according to MacDonald's biographer, David Marquand. After being assured there was "no pact" with the Liberals, Labour's wary National Executive (NEC) voted 16-3 in favour of electoral reform, as did the Parliamentary Labour party.

As now, most (not all) Tories opposed PR. They feared their party would be permanently kept from office by the kind of "progressive alliance" for which Tony Blair and Paddy Ashdown campaigned until Blair's 1997 landslide made it politically impossible. They used the same "progressive" language deployed in 1906-10, when the infant Labour party supported a Liberal government in sweeping social reforms not matched until Attlee's first Labour majority: 1945-51.

In the horse-trading of 1930, an AV bill was introduced and won the support of the Lib-Lab Commons. But it faced crippling amendment in the Tory-dominated Lords and was finally swept away in the sterling crisis of August 1931, when Britain finally abandoned the gold standard which had underpinned a century of its world dominance.

MacDonald then split his party by agreeing to lead an all-party "national government", the kind of coalition formed in the existential crises of both world wars.

In the confusion that followed, the close-fought "who governs Britain?" election in February 1974, Heath found himself unexpectedly defeated. After consulting his cabinet he sounded out the Liberal leader, Jeremy Thorpe, in the hope of cutting a deal that would keep him in power. The Conservatives had polled 226,564 more votes than Harold Wilson's Labour, they had lost 33 seats and were left with 297 to Labour's 301.

It was the third time in Britain's historic first-past-the-post system that the losing side had won more votes, but fewer seats; Labour lost that way in 1951, the Tories in 1929. But the 1974 election sensation was formed by the Liberals – an early version of "Cleggmania". Their vote leapt from 2m in 1970 to 6m; a 19.3% share on a positive swing of 11.8%. Both bigger rivals suffered negative ones.

The Liberals were duly declared the "moral victors" over a discredited two-party duopoly. It was their first real surge since Labour squeezed them to the margins in the 20s. But 4m extra votes gave them only 14 MPs instead of six.

When the two leaders finally met, Heath offered Thorpe three options: loose co-operation bill-by-bill, full consultation on the Queen's Speech programme, or (Heath's preferred option) a formal coalition, including ministerial posts. Brown or Cameron may face similar choices.

Rival memoirs dispute some details. Heath claims that Thorpe was keen to become home secretary, though No 10 already knew of the sex scandal that would break publicly in 1976, destroying the Liberal leader's career. Thorpe, still alive today at 80, insists no specific post was mentioned, but that minister for Europe was in Heath's mind.

In any case, news of the supposedly secret weekend meetings leaked. Predictably, Liberal MPs and activists reacted with alarm at the very thought of keeping an unpopular – and defeated – government in power. The parallel in 2010 is obvious.

Crucially, Heath would concede no more than a Speaker's conference to examine the merits of electoral reform – as ever, the Liberal Holy Grail. It was not enough.