Sir Vince Cable is expected to be confirmed as Liberal Democrat leader later.
Nominations for the contest close on Thursday.
The leadership contest came after Tim Farron announced he would quit because he had been unable to reconcile his Christian faith with the demands of leading a "progressive, liberal" party.
All of the party's other senior MPs have previously ruled themselves out of the race to succeed Mr Farron.
Declaring his candidacy on the Lib Dem Voice website last month, Sir Vince, 74, said he is ready to "work with like-minded people in other parties" to ensure voters get the final say on any Brexit deal, with the option to stay in the EU if the agreement on offer is not good enough.