In politics, trajectory is everything. Had David Cameron achieved a majority of 50 seats in 2010 and seen it reduced to 12 on 7 May, he would be viewed as a dangerously weakened prime minister. Instead, since he previously had no majority at all, he is regarded as immeasurably strengthened. Tony Blair cut a haunted figure after his advantage was reduced from 167 seats to 66 in 2005. When Cameron, dressed in an open-necked shirt, greeted journalists at Downing Street’s summer drinks party, he looked supremely relaxed.

If the prime minister is at ease it is partly because he will not face the electorate again. His unexpected victory has ensured that history will record him as one of the most significant post-war Conservative PMs. Unlike Blair, he faces no immediate internal threat to his status. His Chancellor and likeliest successor, George Osborne, is his greatest ally, rather than his greatest foe.

But the slightness of Cameron’s majority (the smallest of any government since October 1974) imposes constraints on his rule. Already, in the supposed grace period, his will has been thwarted several times. The Conservatives’ plan to replace the Human Rights Act with a British Bill of Rights was postponed after a threatened rebellion by the “Runnymede Tories” (civil libertarians such as former shadow home secretary David Davis and former attorney general Dominic Grieve). A promised vote on English votes for English laws was shelved owing to opposition from the Democratic Unionist Party and Conservative backbenchers. The planned attempt to liberalise the Hunting Act was withdrawn after the SNP vowed to defeat it. “This is a regime that runs away from the sound of gunfire,” one source told me. It is to Cameron’s fortune that the Tories are united on the need for further spending cuts.

The difficulty that the Conservatives will have in reconciling their election pledges (some of which they assumed would be traded away in coalition negotiations) with fiscal reality has already been demonstrated. A promised £72,000 cap on social care costs was delayed for four years owing to a dearth of local government funding. Osborne has already postponed his planned budget surplus by a year to 2019-20. Some Tories privately predict that deficit reduction will become a three-term project if the economy underperforms.

The forthcoming EU referendum, which will define Cameron’s second term, will unavoidably rupture his party. Boris Johnson, Osborne’s chief leadership rival, is gravitating towards the “Out” campaign. As Cameron’s premiership continues, the battle to succeed him will disrupt the efficient operation of government.

But in defiance of these obstacles, the Tories display a serene confidence that they will remain in office after the next election. Labour has 34 more seats than the Conservatives had in 2005, but is in a much less favourable position. Planned boundary changes, if passed, will increase the Tories’ majority to around 50. The SNP’s hegemony in Scotland magnifies Labour’s task in England, where there are just 25 ­marginals (23 Conservative-held) with majorities below 3,000. The Greens and the Liberal Democrats compete for left-leaning support. Austerity and the post-crash economy are creating a more Tory-inclined electorate. There are fewer people employed in the public sector than at any time since comparable records began in 1999. One in seven people (4.5 million) now work for themselves. Many of these voters are receptive to Osborne’s promise of a “higher wage, lower tax, lower welfare” economy.

The Conservatives’ optimism, however, derives less from their strength than Labour’s weakness. They are untroubled by all of the party’s putative leaders. The only candidate they briefly feared, Liz Kendall, is set to finish fourth. Should Andy Burnham win, they will charge him with negligence over the Mid-Staffs scandal as shadow health secretary and with negligence over the economy as chief secretary to the Treasury. Should Yvette Cooper win, they will exploit her refusal to apologise for the pre-crash deficit. Should Jeremy Corbyn win, CCHQ may well dissolve itself.

The rebellion of 48 Labour MPs against their party over the welfare bill enhanced the Tories’ confidence. Ed Miliband’s successor will face continual pressure from the left to resist cuts in defiance of public opinion. At the top of Labour, there is despair over the oppositionist mindset of many new MPs (18 of whom voted against the bill). “They aren’t interested in reaching out,” one shadow cabinet minister told me. Social media is judged to have exacerbated the problem by creating the illusion of mass support. A thousand retweets (just 1/65th of the average constituency) is the digital equivalent of piling up wasted votes in a safe seat.

Rather than merely exploiting Labour’s weaknesses, the Conservatives are addressing their own. Cameron and Osborne recognise that, as Tony Blair wrote after the election, “The country defaulted to the Tories. It didn’t desire them.” They are seeking to shed their reputation as the party of the privileged. Rash moves such as cutting the top rate of income tax have been avoided. Osborne’s promised £9 “National Living Wage” may feel short of the level recommended by campaigners but it is still a landmark intervention by a party that once rejected any minimum. Robert Halfon, the Tories’ creative deputy chairman, is leading a review aimed at transforming them into “the modern trade union movement for working people”.

Defeats, scandals and U-turns likely lie ahead for Cameron. But they were a feature of the last parliament, too. The Tories’ impenetrable confidence is born of the belief that Labour will be in no state to take advantage of them. As they depart Westminster for the summer recess, their opponents fear they are right.