SACRAMENTO — A California initiative to increase the tax on tobacco to pay for cancer research has failed by less than a percentage point after remaining too close to call for more than two weeks.

With about 5 million votes cast, ‘‘no’’ on Proposition 29 led by about 27,000 votes with about 105,000 left to count. The Associated Press analyzed areas where the uncounted votes remain and determined Friday there were not enough places where ‘‘yes’’ was winning to overcome the deficit.

The plan to add $1 to the cigarette tax was led by cyclist Lance Armstrong, a cancer survivor. Tobacco companies, led by Philip Morris, poured millions of dollars into an ad campaign that whittled away support. Polls showed approval peaked around two-thirds in March but had fallen dramatically by the June 5 balloting.

Support for the initiative was strongest in the San Francisco Bay Area, while more conservative places such as Southern California’s Inland Empire opposed it.

Proponents said they would be back.

‘‘This came so close, I think this is worth another try,’’ said Stan Glantz of the University of California, San Francisco’s Center for Tobacco Control Research and Education. ‘‘I think it would be horrible if Philip Morris and Reynolds get away with this.’’

He suggested tobacco foes might turn to the Legislature, though lawmakers routinely reject attempts to raise tobacco taxes.

The opposition campaign will wait until all the votes have been counted before declaring victory, spokeswoman Beth Miller said.