PESHAWAR, Pakistan - From the volatile tribal northwest to the wealthy farm land of Punjab, Pakistan's main opposition leaders geared up their election machinery on Wednesday to challenge President Pervez Musharraf.

After a vote boycott drive disintegrated, both former prime ministers Benazir Bhutto and Nawaz Sharif, back from years in exile, planned to meet party workers and hold public meetings as a campaign clouded by worries of vote rigging gathered momentum.

With the main opposition parties adding some credibility to the election by agreeing to run, political leaders were oiling their party machinery before the campaign picks up pace after the publication of candidates lists on Sunday.

Bhutto planned to meet her party candidates in the northwestern town of Nowshera near Peshawar, capital of the restive North West Frontier Province where in some tribal areas militants are battling government forces.

Sharif planned three public meetings in Punjab, Pakistan's most populous province, which returns about half the members of parliament and is his traditional stronghold of support.

"These meetings are kind of warm-up matches," said Ahsan Iqbal, spokesman for Sharif.

The election is essentially a three-way contest between the two main opposition parties and the Pakistan Muslim League (PML), which backs Musharraf and plays up the strong growth Pakistan has enjoyed under its rule.

The election comes amid opposition fears there is too little time before the January 8 election for a free and fair vote and that the result will be biased in favour of parties loyal to Musharraf, raising the prospect of a contested result.

"Even by Pakistan's own standards of inefficiently managed, chaotically contested and not-so-fair elections, Election 2008 promises to be a skewed affair," wrote political analyst Nasim Zehra in The News newspaper on Wednesday.

Any contested result would lead to the prospect of more instability in the nuclear-armed U.S. ally and likely hurt its efforts to fight militancy.

CONSIDERING COALITIONS

The PML, formed to give Musharraf a political base after his 1999 coup, could fare badly as his popularity has slumped this year amid his efforts to replace a Supreme Court chief seen as hostile to his government.

The election is crucial for Musharraf, who has promised that this weekend he will lift emergency rule, imposed in November.

A poor showing in the election could mean a hostile parliament that might even move to impeach Musharraf, who stepped down as army chief last month, over accusations he acted unconstitutionally in securing a new term as president.

Sharif's trip to Punjab came in what is expected to be a hard campaign battle by his Pakistan Muslim League (Nawaz) against the pro-Musharraf PML.

Bhutto's main strength is in the Sindh province in the south.

Parties were likely to have to try to build coalitions after the election that could produce a hung parliament.

Bhutto has said she expected her Pakistan People's Party will have to enter into a coalition to create a ruling majority, The Washington Times reported on Wednesday.

Bhutto said joining with the party of Sharif would be possible but ruled out an alliance with those controlled by Musharraf or Muslim clerics, the newspaper reported.

"No one will accept a (pro-Musharraf PML) victory," she said.

Sharif has been barred from running because of criminal convictions he says were politically motivated. He has said he has no plans for an electoral alliance with Bhutto.