By Ju-min Park and David Chance
SEOUL (Reuters) - With a week until a deadline to finalize nominations for South Korea's December 19 presidential election, neither of two main opposition challengers appears willing to step aside, which will likely mean the conservatives retain the country's most powerful office.
Moon Jae-in, who has been nominated by the main left-of-centre opposition party, has held talks with independent Ahn Cheol-soo over a joint platform that could lead to a single candidate, but the talks have been bedeviled by infighting and leaks, with neither side appearing willing to give way.
"If I yield arbitrarily, it would be equivalent of breach of trust (for my party)" Moon said on Monday.
Speaking within minutes in the same building, Ahn declined to be drawn on whether the two sides could agree on a single candidate.
Opinion polls show that either Moon or Ahn would stand a chance of beating conservative Park Geun-hye in a straight fight. But a split vote see Park into the presidential palace.
Park is the daughter of dictator Park Chung-hee who ruled the country for 18 years until he was killed by an assassin. Park's mother was assassinated earlier, in an attack backed by North Korea.
Ahn told journalists on Monday Park could be defeated if the backers of both challengers joined forces.
"If we are able to get support from those people who support both camps, I am confident the likelihood of defeating candidate Park is very high," Ahn said.
"But if not, I expect there will be a very difficult battle."
Ahn, a software mogul turned academic and philanthropist, has promised a break from South Korea's confrontational party politics. But the man who once led in opinion polls has seen his ratings slide sharply.
A poll by the East Asia Institute showed Moon was 15.3 percentage points ahead of Ahn on the question of which one should be the sole opposition candidate.
Moon, a former human rights lawyer and confidant of ex-president Roh Moo-hyun, was elected to parliament for the first time in April. Ahn has never held political office.
Facing them is the 60-year old Park, a veteran politician from the conservative Saenuri party, which now holds the presidency and controls parliament.
(Reporting by Ju-min Park and David Chance; Editing by Robert Birsel)