ISLAMABAD, Pakistan -

Pakistani prosecutors said Tuesday they are pressing ahead with corruption cases against opposition leader Nawaz Sharif, a move his lawyer called politically motivated as the presidential race grew increasingly testy.

Asif Ali Zardari, head of the main ruling party and widower of slain ex-Premier Benazir Bhutto, is expected to easily capture legislators' votes for president Saturday. Sharif's party is fielding a retired judge as its candidate.

The election and surrounding political maneuvering are being watched carefully by the United States, which considers Pakistan's cooperation critical in the war on terror.

In mid-August, longtime U.S. ally Pervez Musharraf was forced to quit the presidency by a united ruling coalition that included Sharif's party. Sharif switched to the opposition after disputes with Zardari over who should succeed Musharraf and how to restore judges the former president sacked last year.

Both Zardari and Sharif have been saddled with corruption allegations over the years, but Zardari has seen cases against him vanish in recent months, thanks in large part to a deal struck with Musharraf to pave the way for Bhutto's return to the country.

Zulfiqar Ahmed Bhutta, a top prosecutor with the National Accountability Bureau, confirmed Tuesday that it moved late last month to challenge a court decision to indefinitely adjourn a set of cases against Sharif, also a former prime minister.

The cases stretch back years, and their accusations against Sharif include money laundering, loan defaults and accumulation of wealth beyond his known sources of income.

"It appears to be aimed at using the accountability courts against Nawaz Sharif," said the opposition leader's lawyer, Khwaja Haris. "It appears to be used for political ends."

Zardari has garnered the support of several political factions and parties in recent days, making him a virtual lock for the presidency. His party aides are calling him the "consensus candidate," even though Sharif's party, the second largest, opposes him.

Information Minister Sherry Rehman, a top Zardari party member, insisted Tuesday there was no need to "put any pressure" on Sharif's party in the race.

Zardari has stunned many by becoming the country's most powerful politician, despite no elected position. For years, Zardari has been best known as Bhutto's husband and derisively called "Mr. 10 Per cent" in a reference to the corruption allegations.

Zardari took over the leadership of the Pakistan People's Party after his wife's assassination in December. That party, along with Sharif's, soundly defeated Musharraf's allies in February parliamentary elections, forming a fragile coalition that has come undone.

Though at times he has tried to be conciliatory toward Sharif, Zardari on Monday -- without naming Sharif -- said the former two-time premier was "still immature in politics."

"You need a bit more experience," Zardari said. "So, we are thinking and we are trying and we will wait so that he can enhance his political understanding."

Zardari's party is generally considered more in line with American policies on fighting Islamist extremism than Sharif's, which has a more conservative base.

But many Pakistanis consider Musharraf's support for America's foray into Afghanistan as the reason for rising violence in their country. Such sentiments have led Zardari's party to tread carefully in fighting insurgents.

The government at first tried peace deals with militants, but more recently resorted to force, staging an offensive in the Bajur tribal region that killed hundreds and displaced hundreds of thousands. The military campaign has halted for the Muslim holy month, Ramadan.

On Tuesday, Prime Minister Yousuf Raza Gilani said the government would pursue militants wherever they challenge the government's authority.

"We are not a slave of America," Gilani told legislators. "We are not bound to America, we are bound to our people. Protecting the life and property of citizens is the prime duty of the government."