Rishi Sunak has surged ahead in the race to be Conservative leader while Boris Johnson was mounting a last-ditch scramble to shore up support among MPs, amid warnings his return as prime minister would lead to a political crisis within a week.

As more senior party figures cautioned that a Johnson comeback would lead to chaos and an early election, Sunak won the support of 150 MPs – just shy of the number needed to keep all but one other rival candidate getting on the slate.

Sunak launched his official campaign with a declaration that “fixing the economy” was his priority but he gave no media interviews or formal manifesto showing his proposed programme for government.

Penny Mordaunt, the third candidate, has also launched with about 27 backers, and gave an interview to the BBC’s Laura Kuenssberg saying she wanted to be a “halfway house” between Johnson and Sunak.

In contrast, Johnson had still not even formally declared he was running by Sunday evening. He had just 60 publicly revealed supporters, despite the claims of his allies to have reached the threshold of 100 Tory MPs needed.

Asked if Johnson had made it, one of his closest political friends said: “Yes. Verified.”

A leaked WhatsApp message from his campaign manager, Chris Heaton-Harris, told supporters: “I can confirm we have completed all paperwork to be on the ballot tomorrow.”

But MPs close to the rival camps of Sunak and Penny Mordaunt said they did not believe these claims. One supporter of Mordaunt said: “We think it’s all bluffing as usual.”

Johnson has gained the support of seven cabinet ministers, including James Cleverly and Nadhim Zahawi, but despite gaining some momentum on Friday and cutting short his Caribbean holiday, he only added eight new public backers over the weekend.

Johnson’s supporters argued that many of his backers do not need to reveal their names in public because MPs submit their endorsement to the 1922 Committee directly and can do so anonymously.

Doubts over his confidence were also fuelled after he sought deals with Sunak and Mordaunt, urging them to pull out. Both declined to do so. Johnson’s camp was also circulating a briefing note purporting to show polls suggesting their candidate was electorally more popular than Sunak.

While Johnson battled to get support, Sunak added dozens more Tory MPs to his name on Sunday, from Suella Braverman and Kemi Badenoch on the Eurosceptic right of the party to former remainers such as Stephen Crabb and Grant Shapps.

His success with MPs means he could win outright in the first round of the contest on Monday if no other candidate gets more than 100 supporters. If Johnson or Mordaunt were to get on the ballot as well, there would be a vote of about 150,000 Tory party members to be concluded by Friday.

Prominent Conservatives warned on Sunday that if members opt for Johnson as prime minister again then it could spell disaster for the party. Many MPs are also concerned about the upcoming privileges committee inquiry into whether Johnson misled the Commons over the Partygate scandal – although some sources close to Johnson tried to claim that Sunak could also find the inquiry difficult to navigate.

George Osborne, the former chancellor, cautioned that Johnson would fail to command the support of the parliamentary party, even if he had the backing of the members.

“If Boris Johnson wins, gets into the contest, wins the members’ ballot, I think there’s a real chance the Tory parliamentary party will say: ‘We don’t accept the result of the members’ ballot, we don’t accept that 200 of us are going to serve under a prime minister we didn’t want’ … And so I think the crisis will develop sooner than the privileges committee. I think it will develop at the end of this coming week, and the beginning of next week,” he told Channel 4’s Andrew Neil Show.

Steve Baker, a former Truss supporter, minister and influential figure in the European Research Group faction, gave his backing to Sunak and described a comeback by Johnson as a “nailed-on disaster”.

Baker told Sky’s Sophy Ridge on Sunday: “I think it would be for the best if Boris did something big and statesmanlike. I mean, if he wants to come back as prime minister, he would need to do it after this privileges issue is settled. I think he’d make an amazing chairman of the party …

“But what we can’t do is have him as prime minister in circumstances where he’s bound to implode, taking down the whole government with him. We just can’t do that again.”

His view was echoed by Robert Jenrick, a former cabinet minister. Speaking to Times Radio, Jenrick said: “I cannot in good faith recommend to my parliamentary colleagues and to members of the party that we go from having the shortest prime minister in political history to the first to be expelled from the House of Commons.”

With nominations due in by 2pm on Monday, all eyes will be on whether one or both of Johnson or Mordaunt pull out of the race in the morning.

Mordaunt told the BBC’s Laura Kuenssberg on Sunday that she believed she could bring the party together, but would not outline any economic plans other than keeping the current chancellor, Jeremy Hunt. She denied she had offered Johnson’s camp her support in return for a job – and said she was not contemplating backing another candidate.