Sharon, crucial to Bush's hopes of pushing ahead with the Middle East peace process, was in a hospital after sustaining the stroke in Jerusalem and there were concerns about whether he would be able to recover.
"Laura and I share the concerns of the Israeli people about Prime Minister Ariel Sharon's health, and we are praying for his recovery," Bush said in a written statement.
"Prime Minister Sharon is a man of courage and peace. On behalf of all Americans, we send our best wishes and hopes to the prime minister and his family," Bush added.
In a separate statement, Secretary of State Condoleezza Rice said: "Our thoughts and prayers are with Prime Minister Sharon, his family, and the Israeli people. We wish the Prime Minister a full recovery."
White House national security adviser Stephen Hadley briefed Bush on Sharon's condition and National Security Council spokesman Frederick Jones said officials were monitoring the situation.
Bush has relied heavily on Sharon, as he attempts to coax Israelis and Palestinians into a peace agreement.
He scolded the hefty Sharon in December to watch what he eats and get more exercise after the Israeli leader, 77, had an earlier stroke.
Sharon's health crisis, combined with the possibility of a delay in the January 25 Palestinian election, will further slow Bush's long-sought quest for two states, Palestine and Israel, living in peace, analysts said.
"I don't think we're going to have any efforts for a while," said Edward Walker, president of the Middle East Institute and a former US ambassador to Israel. "I don't see how you can marry up the Palestinians with the Israelis when both are undergoing leadership crises."
"I don't see how you can marry up the Palestinians with the Israelis when both are undergoing leadership crises"
Edward Walker, president of the Middle East Institute
Bush considers personal relationships important and had developed one with Sharon, but experts noted he has meager ties with other Israeli officials such as Sharon's deputy, Ehud
Martin Indyk, director of the Saban Center for the Middle East at Washington's Brookings Institution who is also a former US ambassador to Israel, said Sharon "will leave giant shoes to fill" if he is forced to leave the political scene.
Indyk and other experts said Israeli elections should go forward in March even with the leadership crisis.
If Sharon's centrist Kadima Party wins, it will help Bush's policy, but if rightist former prime minister Benjamin Netanyahu emerges victorious, any disengagement from the West Bank would likely be put off.
If that happened, said Indyk, the Bush administration would "resort to its default position, which is to put the Israeli-Palestinian issue on the back burner and focus on issues that are seen to be more critical."
"Bush's policy in the Middle East depends far more on what happens in Iraq than on what happens in Israel," Indyk said.
James Zogby, president of the Arab American Institute, said he doubted US policy would change with Sharon's passing from the political scene.
"I don't think the administration's going to do anything different. The president learned early on that if he said two states and did little or nothing to make it happen, it would
buy him time.
"I think the administration will find a way to buy time. They've done it for five years, they'll do it for three more," he said.