Rebecca Long-Bailey has blamed “mismanagement and a bad organisational culture” for contributing to Labour’s general election defeat, promising to “professionalise” the party if she becomes its next leader.

In what appeared to be a veiled criticism of cronyism under Jeremy Corbyn, Long-Bailey promised that under her leadership, “promotions will be based on what you know, not who you know”.

“I don’t care which wing of the party you’re from. If you’re competent, professional and get the job done, I want you working for Labour,” she added. She said the party needed to be prepared to “go toe to toe with Dominic Cummings” – Boris Johnson’s key adviser.

Long-Bailey declined to mention any individual by name. Labour’s campaign was overseen by close allies of Corbyn, including his former chief of staff, Karie Murphy, since nominated for a peerage; the Labour party general secretary, Jennie Formby; and the director of strategy and communications, Seumas Milne.

The leadership contender highlighted several shortcomings in Labour’s campaign, including its digital output, and the way party resources were targeted. Key digital tools were starved of funding and often broke down, hampering activists’ ability to campaign, she said.

“While Jeremy’s social media pages built on their 2017 success, despite staff’s hard work the official Labour party operation fell back in some areas and didn’t match up to the Tory strategy of pumping out tailored, persuasive content.”

She also claimed the party’s approach to allocating campaign resources was ineffective. “Targeting the wrong seats based on partial data meant activists wasted efforts campaigning in places we couldn’t win,” she said.

During the 2017 campaign, Corbyn’s team claimed Labour officials who had worked for the party since the Ed Miliband era were too cautious about pouring resources into attacking seats.

This time, however, Labour began by targeting 96 constituencies, 66 of which were attacking seats. They gradually shifted towards a more defensive stance as the campaign went on; but many Labour MPs hanging on by slim majorities complained that they did not receive enough support.

In the event, more than 50 lost their seats – while campaigners were still pouring into seats including Boris Johnson’s Uxbridge constituency, which with hindsight appeared unwinnable.

Long-Bailey said she would “throw open the doors of HQ” to Labour’s 500,000-plus members, and “have open days where members and staff can exchange ideas and work alongside each other and move as many operations as possible outside of London”.

Long-Bailey has the tacit backing of Corbyn, though he has not declared his public support for any candidate.

Long-Bailey has 58 nominations to Starmer’s 138, according to the LabourList website, which is keeping a tally of local meetings.

But she has the support of Corbynite Labour campaign group Momentum, which is encouraging its members to “Ring for Rebecca”. Momentum’s co-founder, Jon Lansman, is advising her.

Long-Bailey, the MP for Salford and Eccles, declared recently that she would mark Corbyn 10/10 for his time as leader; but she has also sought to distance herself from aspects of his leadership, including by promising to include a broader spectrum of MPs on the party’s frontbench.

She has previously suggested Labour needs a more “aspirational” message to voters.

Long-Bailey, Starmer and Wigan MP Lisa Nandy have all secured enough support from trade unions and local parties to make it onto the ballot paper. Emily Thornberry is struggling to do so, with just four CLP nominations of the 33 she needs before the 14 February deadline.

The result of Labour’s leadership contest will be announced at a special conference in London on 4 April.