Emily Thornberry said she would “frighten the life out of” Boris Johnson if she became the next Labour leader.
In a pitch to party members, the shadow foreign secretary accused the prime minister of having a “woman problem” and said she is the best candidate to take him on regularly in sparring matches in the House of Commons.
Her comments come in what is proving to be a showdown week for the contenders, after shadow Brexit secretary Keir Starmer became the first to be guaranteed a place on the final ballot paper.
He won the nomination from retail union Usdaw on Monday to add to his earlier shows of support from Unison and the Socialist Environment and Resources Association (Sera), an affiliate group.
The other four candidates are pitching to win the backing of the remaining big unions, which include Unite, GMB and the Communication Workers Union (CWU).
Unite’s close links to the left of the party are said to make shadow business secretary Rebecca Long-Bailey the favourite for its tip.
Lisa Nandy is being touted as the front-runner for the support of the GMB, with its executives due to meet in London on Tuesday to decide which of the five candidates to rally behind.
However, even if GMB does decide in Nandy’s favour, the Wigan MP would still require another nomination from an affiliated group to get her through to the last round.
Candidates are required to have won the nomination of three Labour affiliates, including at least two unions, which amount to at least 5% of affiliate members.
The only other route on to the ballot paper is by receiving nominations from at least 5% of constituency Labour parties (CLPs), meaning Thornberry and Jess Phillips face an uphill battle to ensure they continue in the race.
If they fail to win the support of a sizeable union, then they will need the nomination of 33 CLPs instead.
Thornberry was defiant about her chances, telling ITV’s Good Morning Britain she remains “in this to win it”.
Asked who she would vote for out of rivals Starmer and Long-Bailey if she did not make the final shortlist, she replied: “I’m not getting into this. I’m in this to win it.”
The Islington South and Finsbury MP argued that the party should be looking to elect a woman of her experience and working-class background to the top job.
“It is an advantage to be a woman leader at this time because I think Boris Johnson has a woman problem, most definitely,” she told the programme.
“He certainly has a problem with me. I think the Labour Party should think about that.”
She pointed out that Labour now has more women MPs than male representatives in Parliament, and said she is the candidate who would “frighten the life out of” Johnson at future Prime Minister’s Questions.
All five contenders have agreed to take part in a televised debate on Channel 4 next month.
The hour-long programme will be hosted by Krishnan Guru-Murthy, with a live audience asking questions.
It will take place on Monday 17 February at 8pm – only a few days before members and others start to cast their vote.
The Labour membership surged, according to the Mirror, to more than 540,000 people ahead of Monday evening’s sign-up deadline.
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