Earlier negotiations with Pervez Musharraf, the Pakistani president, could see the two rivals team up to form a US-friendly alliance.
Those talks have already prompted Musharraf to issue an amnesty covering the corruption cases that made her leave Pakistan in 1999.
Authorities have warned that Bhutto, who has said that if elected she will co-operate with the American military in targeting Osama bin Laden, could be targeted by al-Qaeda fighters.
The government of Sindh province, of which Karachi is the capital, appealed to Bhutto to abandon plans for a 16km procession into Karachi, saying it would leave her vulnerable.
"We have informed Ms Bhutto and her team of the situation and advised them to cut short the programme instead of going for a 18-20 hours-long procession as this would be tantamount to inviting trouble," Ghulam Muhammad Mohtarem, Sindh's home secretary, said.
But Bhutto dismissed such fears, saying on Wednesday: "I don't believe that a true Muslim will make an attack on me .... Islam forbids suicide bombings."
Karachi is expected to be paralysed by the procession, but fears of a repeat of clashes that left more than 40 dead in the city in May have eased.
The pro-Musharraf Mutahida Qaumi Movement said it would not obstruct Bhutto when she arrives.
But the former prime minister's political future, and her possible alliance with Musharraf, will ultimately depend on her party's showing in the elections.
"Pakistan needs a military leader who can control both civil and possible military extremism"
Creative_person01, Islamabad, Pakistan
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Musharraf, who won a vote by lawmakers on October 6 to give him a new five-year presidential term, has seen his popularity plunge in recent months.
His deal with Bhutto appears aimed at boosting his political base as he tries to extend his rule.
Meanwhile, Pakistan's highest court on Wednesday heard challenges to the legality of Musharraf's re-election.
Opposition parties, which had boycotted the presidential vote, argued it was unconstitutional for an outgoing parliament to choose a new president and that Musharraf should have been disqualified under a ban on public servants seeking elected office.
It is unclear when the nine-member panel of judges will rule on the case, which has injected more uncertainty into Pakistan's already turbulent politics.
The supreme court is also examining the legality of the amnesty, signed by Musharraf, that quashed corruption cases against Bhutto.