Like his predecessor, Pervez Musharraf, a strong US ally but an unpopular figure who resigned in August, Mr. Zardari has expressed strong support for fighting terrorism. But his record on democratic reform is less convincing so far.
Without such reforms, which would weaken Zardari's powers as president, "the anger which was against Musharraf will be against Zardari," says Khalid Rahman, an analyst at the Institute of Policy Studies in Islamabad. America does not want Pakistan to return to the volatility of Mr. Musharraf's last year in power.
With Musharraf gone, US officials have focused more on establishing ties with the Pakistani Army than with its new civilian government. Chairman of the US Joint Chiefs of Staff Adm. Mike Mullen has met Army chief Gen. Ashfaq Kayani five times since October.
From the perspective of US policymakers, Zardari and the two other candidates are unlikely to fill the void left by Musharraf. For much of his tenure, Musharraf was both president and chief of Army staff, meaning he united in one man what the Bush administration sought most: the means to act against terrorists and the political will to do it. Musharraf's successor will no longer wear both hats.
As the security in Afghanistan has steadily deteriorated, pressure has mounted for US forces to play a greater role in antiterror efforts in neighboring Pakistan, where many of the terrorists seek sanctuary.
Admiral Mullen's meetings with General Kayani suggest that the Pentagon is trying to cudgel the Pakistanis into greater action – and that it sees its greatest post-Musharraf ally as Kayani, not a president or prime minister.
Indeed, Wednesday's strike in South Waziristan, which was the first time American ground forces are alleged to have led an operation on Pakistani soil, suggests that Musharraf's departure has left a political void.
Musharraf kept tighter control of the government than the civilian government has, says Mr. Rahman. "That has made the job of Americans easier to take their liberty," he adds, citing the attack.
There is little suspense going into Saturday's election because Pakistan's president is not chosen by the people. Instead, he is elected by the four provincial assemblies and the national parliament. Zardari's Pakistan People's Party (PPP) – formerly led by his wife, the late Benazir Bhutto – and its allies have a collective majority there. For this reason, his two competitors – a former judge, Saeeduzzaman Siddiqui, and former journalist,Mushahid Hussain Sayed – do not appear to have the votes to challenge Zardari.
Still, the perfunctory feel of Saturday's election belies the importance of the job. If he wins Saturday, Zardari will inherit a post far more important than the ceremonial position the framers of Pakistan's Constitution intended.
Pakistan's past military rulers – including Musharraf – all installed themselves in the presidency and made constitutional amendments to give the office extraordinary powers, such as the ability to dissolve parliament.
For decades, political reformers – including Ms. Bhutto – have said that this puts too much power in one man, undermining the checks and balances of Pakistani democracy.
February's ballot-box revolution has given reformers their chance to return parliament to its original preeminence. The forces that supported Musharraf's one-man rule were routed and the results overwhelmingly sanctioned a collaborative coalition government.
The question now is whether Zardari and his PPP, which leads the governing coalition in parliament, will follow through. PPP officials have said they will. But Zardari's rapid rise being Bhutto's husband to being head of Pakistan's largest political party and presumptive president has raised concerns about his thirst for power and whether it will trump the need to make reforms.
"The prime minister will do exactly what [Zardari] wants him to do," says Shafqat Mahmood, a former PPP senator who now writes a column for The News, a national daily.
This has led to a sense of unease among Pakistanis on the eve of the election. "With the present mind-set he is in, I don't think he would want to give his powers to the prime minister," says Zafar Minhaj, a businessman in Karachi, by telephone. "Zardari is running the show now."
Saying that "we have very limited choice in the present situation," Mr Minhaj calls Zardari: "the best among the worst."
There remains lingering goodwill from the February elections, says Hassan Askari Rizvi, an independent political analyst in Lahore. But "much will depend on how [Zardari] behaves as president," he says.
"Who is seen to be in the driver's seat?" he asks. "Will Zardari try to control everything from the presidency like Musharraf?"
The US strike in South Waziristan will not help, he and others add. If Zardari does not condemn it, he'll be seen as Musharraf was – capitulating Pakistan's sovereignty for political gain of alliance with the US.
"If he doesn't [condemn it] he will be seen as a collaborator," says Mr. Mahmood, the columnist.
Ironically, it comes at a time when Zardari and the civilian government have been making progress in rallying public support for the war on terror, casting it as a Pakistani war, not a proxy war for America. Zardari reiterated that stance in a column in Thursday's Washington Post. "We stand with the United States, Britain, Spain and others who have been attacked," he wrote. "Fundamentally, however, the war we are fighting is our war. This battle is for Pakistan's soul."
With such strikes, which include claims of US involvement and significant civilian casualties, "all the momentum will be lost," says Najmuddin Shaikh, a former foreign secretary of Pakistan.