Liz Kendall has delivered a thinly-veiled swipe at Labour leadership rivals Andy Burnham and Yvette Cooper, warning that the party will lose if it goes for someone who is "comfortable and familiar".
The jibe came as the trio and left-wing backbencher Jeremy Corbyn faced off at the latest hustings in Stevenage.
Mr Corbyn received some of the warmest responses from the audience as he spelled out his "moral" opposition to renewing Trident and lambasted Tory welfare reforms.
Mr Burnham, Mrs Cooper and Ms Kendall, by contrast, were heckled on both topics.
All three were greeted by calls of "shame" as they endorsed a like-for-like replacement of the nuclear deterrent and rejected unilateral disarmament.
Shadow health secretary Mr Burnham insisted: "As Prime Minister of this country the safety of this country must come first."
Shadow home secretary Mrs Cooper said she would like to see a global reduction in weapons stocks, but "in the absence of that I would renew Trident."
Shadow health minister Ms Kendall added: "Some of you won't like it, but I have got to tell you how it is."
The three frontbenchers all avoided condemning outright Tory plans to reduce the benefit cap from £26,000 to £23,000, with Mr Burnham arguing the party could not appear "comfortable" about people receiving more than a normal salary.
Asked how they would handle Tory accusations that Labour's economic mismanagement left Britain vulnerable to the credit crunch, Mr Burnham, Mrs Cooper and Mr Corbyn all denied the previous government could be held responsible for the crash.
Ms Kendall argued that the party needed to focus on how the economy should be handled in the future, rather than dwelling on the "mistakes of the past".
In a parting shot seemingly aimed at her rivals - who both served in the Cabinet in the previous Labour government - Ms Kendall told the activists she represented a "fresh start".
"This is not the time for safety first or turning back to what is comfortable or familiar," she said.
Ms Cooper commented pointedly afterwards that her experience meant she would be "comfortable" taking on the Conservatives from "day one".