Foreign secretary Liz Truss was prepared for former chancellor Rishi Sunak to take her on this week in the television debates about the race to be Britain’s next prime minister, but not with such ferocity.

“For the TV debates we prepared for three scenarios: that he went on the attack, that he was Mr Patronising, or a mixture of both,” said one Truss ally. “We were astonished at how aggressive he was.”

But at the end of a week of bruising political combat between the two candidates to succeed Boris Johnson as Conservative leader, it was Sunak rather than Truss who was on the defensive.

The former chancellor’s attacks on the foreign secretary and his policy interventions — whether on China or tax — were turned against Sunak, leaving him flailing.

“Almost everything he has tried, he has come out worse,” said David Gauke, a former Tory cabinet minister. Bookmakers now have Sunak typically at 5-1 to win the race, with Truss the red hot favourite at 1-8.

Time is running out for Sunak: more than 150,000 ballot papers start arriving on the doorsteps of Tory members on Monday, meaning that voting to choose the next party leader begins next week.

Sunak hopes a series of 12 hustings with party members, which started in Leeds on Thursday, will provide him with a route back. But Truss performed well in her home town and Sunak was accused by one member of “stabbing” Johnson in the back by resigning as chancellor this month.

Sunak’s strategy of presenting himself as the serious leader for difficult times so far appears to have failed. Instead, Truss has portrayed the former chancellor as the continuity candidate, unable to break free from Treasury orthodoxies and leading Britain into recession.

Throughout this week Sunak has tried to close the gap on Truss, notably on Monday during a primetime BBC1 debate when he frequently interrupted the foreign secretary to attack her “inflationary” plans to immediately cut taxes.

Opinion polling after the debate suggested viewers thought the event was essentially a draw, but many viewers complained about the interruptions. Truss’s team accused Sunak of “aggressive mansplaining”.

Then, having spent much of his campaign criticising Truss for proposing to swiftly cut taxes, Sunak on Tuesday announced his own “temporary” £4bn cut in value added tax on household energy bills.

Sunak insisted the move would help to curb inflation, but as chancellor he had previously rejected the VAT reduction, saying it was badly targeted by helping wealthy households as well as the poor.

The Truss team described Sunak’s VAT proposal as a “screeching U-turn”. Johnson, who had wanted to cut VAT but was blocked by Sunak, joked that it now seemed “easier than we thought”.

Attempts by Sunak to present himself as a China hawk also self-destructed, after it transpired on Thursday that he made plans as chancellor for a major UK-China economic and financial conference. The meeting was later scrapped.

Sunak has discovered the problems of running for the Conservative leadership as a former chancellor: the person in government whose job it is to make difficult calls deemed to be in the economic national interest. Hoping to run on his record, Sunak instead appears trapped by it.

Johnson’s allies — who accused Sunak of treachery by quitting the government and triggering the prime minister’s premature exit — have delighted in his difficulties as they seek to thwart his leadership bid.

One Johnson ally said Sunak had “messed up the entire positioning” of his leadership campaign, running as a Leaver backed by some of the party’s biggest Remainers, and claiming to be a tax cutter who had in reality presided over a big expansion of the state as chancellor. “He’s essentially on the wrong side of the party membership on every major issue,” added the Johnson ally.

While Truss has told Tory members about her tough position with Brussels on Northern Ireland’s post-Brexit trading arrangements, Johnson’s supporters highlighted how Sunak favoured a softer stance to avoid provoking a trade war with the EU.

Sunak’s decision to raise national insurance contributions to pay for extra NHS and social care spending — opposed by Truss at the time — is now cited by Johnson’s backers as evidence that he was a “socialist” chancellor.

Truss meanwhile is growing into the Tory leadership contest. After coming across as a wooden automaton in the first TV debate on Channel 4, her human side has come out in subsequent ones, notably recoiling in horror when presenter Kate McCann fainted in a TalkTV event on Tuesday.

The foreign secretary, who campaigned for Remain in the 2016 EU referendum, has learned much from the Vote Leave campaign. She is accusing Sunak of running “Project Fear” by saying her unfunded tax cuts might worsen Britain’s economic plight.

Sunak’s team had hoped that by now opinion polls would suggest that ordinary voters prefer him to Truss. Although YouGov produced a survey this week showing Sunak was more popular with swing voters, the evidence is far from overwhelming.

Conservative MPs backing Sunak are in despair. One said: “The campaign just hasn’t worked well. The campaign started off as a submarine operation, doing as little media as possible and avoiding scrutiny. Then they panicked and they’re having to put him everywhere.”

The former chancellor’s supporters insisted he could still turn things round. They said there were weeks still to run in the contest — the result of the vote by Tory members will be declared on September 5.

Conservative MPs who backed Sunak for the party leadership are urging Tory members to support the former chancellor on the basis he is more likely than Truss to win the next election. “That’s persuasive,” said one MP.

But other MPs who favour Sunak warned that his chances of being the next Tory leader were limited, with the clock rapidly ticking down.

One former minister said: “It’s going to take a big unexpected surprise for Liz not to win it now.”

Another ex-minister said that Sunak’s “not got long. The concrete is beginning to set.”