Owen Smith, who was shadow work and pensions secretary until his resignation last week, is a more likely challenger to Jeremy Corbyn than Angela Eagle, according to a former senior member of the shadow cabinet.
The MP told ITV News that backing for Mr Corbyn "on the ground" is falling every day the Labour leadership drama continues, while Mr Smith is picking up support within the party.
If Mr Smith is considered to have a greater chance of beating Mr Corbyn than Ms Eagle, it may be that he is seen as politically more left-wing than the former shadow business secretary.
Those MPs who oppose Mr Corbyn's leadership say they are in no hurry to give him and his team the contest they want so it can divert attention from "their crisis".
Of the 60,000 new members reported in the last few days, the MP said most are joining to support Corbyn, but only just.
Many, it is claimed, are joining because they want a change of leader.
The belief among Labour MPs is that the right candidate and the right campaign can beat Mr Corbyn - if and when a contest is triggered - even though 60% of Labour's membership voted for him in the 2015 leadership election.
There is still no consensus on when the moment will come to formally challenge Jeremy Corbyn.