ISLAMABAD, Pakistan — Pakistani President Asif Ali Zardari has confounded predictions of his downfall throughout his term in office, but even as the U.S. ally unexpectedly returned to the country this week, many still believe that his demise is close at hand.

Zardari, who's become politically poisonous at home and whose government appears unable to deliver on either domestic or international fronts, came back to Pakistan Monday only to confront a political crisis triggered by the allegations of an American businessman, Mansoor Ijaz. It is the latest and possibly most serious crisis of a presidency dogged by successive political calamities.

The labyrinthine "memogate" scandal sparked by Ijaz has brought the president's health and that of his 4-year-old administration to the brink of collapse, putting the already frozen relations with Washington on center stage once again.

The president was flown to Dubai on Dec. 6 for medical treatment after suffering heart trouble. Many observers forecast at the time that he would never return to Pakistan. Although he has returned, Zardari went to Karachi, not the capital Islamabad, setting off yet new speculation that he wouldn't stay long and could soon go into permanent exile.

Zardari spokesman Farhatullah Babar, sounding exasperated, said Tuesday: "The president is in the country, is here to stay and there are no plans for travel abroad."

The president's focus on political survival means for now that other issues that Washington would like prioritized — an economy in a tailspin, sanctuaries for militants on Pakistani soil and efforts to push Afghan insurgents to the negotiating table — are receiving little attention.

"We know for a fact that President Zardari is a fighter — unwilling to give up even when pinned to the ropes," said an editorial published Tuesday in Express Tribune, a Pakistani daily. "Now that he has squashed the ceaseless speculation, the president's next task will be to take command of the reins of a shaky state."

In an apparent attempt to show that he was back in control, Zardari held a series of meetings Tuesday in Karachi, including with his loyal prime minister, Yousuf Raza Gilani. But the president's troubles are far from over.

This week, Pakistan's Supreme Court took up the memo case, in which Zardari's former U.S. ambassador, Hussain Haqqani, is accused of crafting a "treacherous" missive to the American military's top command. The memo reportedly offered to rein in the Pakistani military in return for more American support for Zardari's weak government.

Ijaz, who runs an investment company and has links to the American military and intelligence establishments, delivered the anonymous message in May. Haqqani denies having a hand in the memo.

While Haqqani was fired over the incident and Ijaz's motives remain unclear, the main target of the allegations is Zardari. The case was brought by opposition leader Nawaz Sharif, and full hearings are expected to begin Thursday.

The evidence in the case appears flimsy, but the dirt surrounding it is politically toxic. The case pits the military, which asked the court to investigate the allegations, against the fragile civilian government, which has called for them to be dismissed. The army has ruled Pakistan for half its existence, and many see the memo affair as a prelude to another military-engineered change in government.