The second round - in effect a re-run of the first ballot - opened at 1pm today, and the 166 Tory MPs have until 5pm to cast a vote.
Michael Portillo, who won the first ballot on Tuesday, is still the favourite to win the initial MPs' voting stage of the contest.
But after casting his vote, he told reporters the voting process had become "unbelievably complex", but would "get real" next week.
He said: "I have never known such complicated arguments for the way people are casting their votes.
"They can tell you they are voting X on round one, Y on round two, Z on round three and somebody else on round four. It's just absolutely amazing what's going on.
"I think probably what happened in round one is that people thought I was ahead and a lot of people therefore voted tactically.
"There will be tactical voting again today. Next week it gets serious. Next week it gets real. We'll see what happens."
However, his hopes of attaining the ultimate goal of succeeding William Hague were dealt a blow today when a Daily Mail poll of Conservative party chairmen rejected Mr Portillo as their choice of leader.
As the shadow chancellor warned his party to "reform or die", the survey of 300 chairmen and party officials gave him just 34% backing.
Today's ballot is an indentikit version of Tuesday's first round because the two trailing candidates, Michael Ancram and David Davis, tied for last place and could not be eliminated.
Now, in the unlikely event that no votes are horse-traded and they tie again, both will be jettisoned from the race. If there is no draw, the last-placed candidate will drop out.
Tuesday's ballot saw Mr Portillo win 49 votes, Iain Duncan Smith receive 39, Kenneth Clarke come third with 36 votes, and David Davis and Michael Ancram tie on 21 votes. All 166 Tory MPs voted, as they are expected to do again today.
The result should be known shortly after 5pm, with the three, or four, remaining candidates facing the same hurdle again next Tuesday.
Momentum, always important in such a convoluted contest, is now behind the right-wing challenger Iain Duncan Smith, after the favourite Mr Portillo failed to win by a decisive margin on Tuesday.
But no pundit is confidently predicting where the 42 votes shared by the two losing candidates will end up.
In a bid to get the sail back in his campaign sails, Mr Portillo today wrote in the Daily Telegraph: "However comfortable the old ways might be, it is necessary to adapt or die. There is widespread recognition in the party that, after two landslide defeats, we have got to be serious about change," he wrote.
"Freedom matters. Personal responsibility matters. But it is no good preaching the eternal truths of Conservatism if the language we use is stuck in a particular time and place - the England of 25 or 50 years ago.
"It is precisely because the principles are eternal that each generation has to be free to express them in their own language."
Once the final two names emerge from the current five contenders, a postal vote through August of the party's 325,000 members will select the leader.
An announcement of the winner will be made on September 12.
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