Leadership contenders David Cameron and David Davis are in the midst of a six week campaign to win a postal ballot of 253,600 Conservative members.
Eleven hustings are being held across the UK. Whoever gains the most votes becomes party leader, with a final result expected on 6 December.
The following day, the winner will face Tony Blair at prime minister's questions in the House of Commons.
The rivals are allowed to spend up to £100,000 each on their campaigns.