Prime Minister Ariel Sharon joked with aides and discussed affairs of state Monday after suffering a mild stroke, but his illness raised questions about his ability to lead the country if elected to a third term in March elections.

It also left his brand-new centrist Kadima Party scrambling. Without the 77-year-old Sharon, Kadima wouldn't likely amount to much, as the prime minister's popularity is the overriding factor behind the party's commanding lead in polls.

With balloting three months away, concerns about his health could become a focus of the campaign, and improve the prospects of the hardline Likud Party that he quit to form Kadima.

Likud voters went to the polls Monday to choose a replacement for Sharon as party leader. Polls gave former Prime Minister Benjamin Netanyahu a slight edge over Foreign Minister Silvan Shalom.

Sharon underwent additional tests Monday after being rushed to Hadassah Hospital in Jerusalem on Sunday night, showing signs of confused speech.

Israeli media reported he was unable to tell time or count fingers when he underwent preliminary neurological tests at the hospital, but doctors later said he improved quickly.

Sharon was treated with blood thinners and suffered no damage from the stroke, said his personal doctor, Boleslaw Goldman.

Hospital spokeswoman Yael Bossem-Levy said doctors ran more tests Monday and decided Sharon should also undergo brain and full body scans, procedures she described as routine.

Sharon held his daily staff meeting in the hospital, said aide Asaf Shariv.

"He asked questions, he received an update from the military secretary and from the Cabinet secretary. He's in good spirits," Shariv told Army Radio.

Cabinet Secretary Yisrael Maimon told Army Radio that Sharon walked around his room and showered by himself. Maimon said it was not clear when Sharon would be released.

The Web site of the Haaretz daily reported that one of its reporters spoke to Sharon late Sunday night.

"I'm fine," Haaretz quoted Sharon as saying. "Apparently I should have taken a few days off for vacation. But we're continuing to move forward," he said, making a play on the name of his party, Kadima, which means "forward."

Doctors said Sharon is now more prone to a stroke, but that doesn't necessarily mean another one would be more severe.

Sharon was at risk for stroke because of his age and obesity. He has never released medical records, and a right-wing lawmaker and physician, Arieh Eldad, has demanded that the prime minister do so now.

Sharon received calls from President Bush, Secretary of State Condoleezza Rice, British Prime Minister Tony Blair and Egyptian President Hosni Mubarak. Palestinian leader Mahmoud Abbas also sent wishes for a speedy recovery.

In Gaza, however, dozens of armed men from the Popular Resistance Committees, a small Palestinian militant group, fired guns in the air, screamed "Sharon is dead" and handed out pastries to motorists to celebrate the news that Sharon was ill.

Sharon led Israel's fight against the five-year Palestinian uprising, and many militants still see him as an enemy despite his pullout from Gaza earlier this year.

A leading member of Sharon's Kadima Party insisted that the prime minister's illness wouldn't cripple the party.

"The prime minister's leadership is the cornerstone of Kadima, and will continue to be," Justice Minister Tzipi Livni told Army Radio. "But there is a worthy coterie as well," she said, referring to other ranking members of the party, such as herself and Finance Minister Ehud Olmert.

Sharon formed Kadima after concluding that his efforts to persuade Likud to support peace concessions were to no avail.

On Monday, Likud voters were asked to choose a replacement for Sharon from a field of four candidates: Netanyahu, Shalom, hard-liner Moshe Feiglin and Agriculture Minister Yisrael Katz.

Netanyahu quit Sharon's government shortly before the Gaza pullout, saying he could not be a party to the withdrawal that he had supported in Cabinet and parliament votes. Although he also ceded land to Palestinians when he was prime minister in the 1990s, Netanyahu is now taking a tough line against further territorial concessions.

Shalom is considered more moderate, and more likely to join a coalition government with Kadima should it emerge as the dominant party in March elections.