The nation's leading pollster, Steadman group, gave Odinga 46 percent to Kibaki's 42 percent, compared to a tiny 0.3 percentage points lead the previous week.

Another three surveys gave Odinga 43 percent against Kibaki's 39, two of them showing an increased lead over their last surveys at the end of November.

With Kibaki's Party of National Unity (PNU) spending heavily on its campaign, and Kenyan politics apt to shift quickly, analysts said, however, the vote could be won by either man.

"This is not the time for any campaign to go to sleep. Three weeks is a lot of time in an election," said Steadman analyst Tom Wolfe. "The undecided voters could swing the election."

Measured and non-confrontational in his public persona, Kibaki, 76, contrasts in style with the more voluble Odinga.

But on major policy substance, the pair differ little, both promising to increase Kenya's impressive recent economic growth, create jobs, tackle poverty, and provide free secondary education.

Campaigning ahead of the December 27 presidential and parliamentary ballots have been marred by violence at rallies, including stone-throwing between rival supporters.

Several people have died in melees around rallies.

There have also been allegations of fraud including vote-buying and inflated registration lists.

Western diplomats, who are tracking the election, say a close vote, which passes off peacefully, would boost Kenya's democratic credentials. They also applaud Kibaki's assurance he will step down "and go home" if defeated.

But they fear Kenya's history of violence at election times may repeat itself.

"The fact you that have an election where an incumbent can lose is a sign of political maturity in some ways," one diplomat said. "My greatest fear, though, is the scenario of a close result with the perception on one side that the vote was stolen. That heightens the risk of violence."

The Steadman survey showed Odinga ahead in the capital Nairobi, plus five more of Kenya's eight provinces.

Kibaki was ahead in Central province, the heartland of his Kikuyu tribe, and was tied with a third candidate -- former Foreign Minister Kalonzo Musyoka -- in Eastern province.

To win, a presidential candidate must have a majority of all votes cast, as well as a minimum of 25 percent in five out of Kenya's eight provinces.

Wolfe said voters in Kalonzo's Eastern province could swing the vote. "Kalonzo voters look like they could go either side. If they vote for Kibaki, Kibaki wins the election. If they vote with Kalonzo, this poll result remains as it is," he said.