BOSTON, Massachusetts (CNN) -- Sen. Ted Kennedy was undergoing more tests Monday as doctors tried to figure out what caused his seizure.
Sen. Ted Kennedy, shown in May 2007, was rushed to a hospital Saturday morning.
"[It's] unclear if we'll know anything conclusive later today or tomorrow," spokeswoman Stephanie Cutter said Monday. "As of right now, I expect him to stay again tonight."
The 76-year-old Democratic icon was flown to the hospital after having a seizure Saturday.
Medical officials this weekend said he seemed to be out of any "immediate danger."
Doctors at Massachusetts General Hospital offered no update Sunday or Monday morning. Watch what might have caused the seizure »
Dr. Larry Ronan, Kennedy's primary care physician, said Saturday that the senator was "resting comfortably and watching the Red Sox game with his family."
Kennedy's family was with him at the hospital, and he received a call from Democratic president contender Barack Obama on Sunday, Cutter said.
Obama told reporters at a campaign stop in Milwaukie, Oregon, that Kennedy "sounded great, he sounded like his usual old self."
"I am just thrilled that Teddy is back on the mend," Obama said. "And I suspect he will be on the floor of the Senate immediately, doing the people's business. And there's not a better senator, or better friend, or better supporter for me than Ted Kennedy."
Kennedy, who ran unsuccessfully for the Democratic presidential nomination in 1980, has endorsed Obama.
Obama's presidential rivals, Sens. Hillary Clinton and John McCain, also expressed thoughts and prayers for Kennedy.
Well-wishers across the country sent messages to Kennedy and his family.
Senate Majority Leader Harry Reid said Saturday he spoke to the wife of the Massachusetts Democrat, Victoria Reggie Kennedy, and she told him her husband is going to be fine.
"Everyone knows he is a strong fighter," Reid said Saturday, speaking at the Nevada state Democratic Party convention.
Initially, a well-informed, prominent Democratic source in Massachusetts had told CNN that Kennedy seemed to have "symptoms of a stroke."
Someone in the Kennedy compound in Hyannisport called 911 Saturday morning, and an ambulance soon rushed Kennedy to Cape Cod hospital. Watch how paramedics' fast response time may have saved Kennedy's life »
He was then flown to Massachusetts General, where he had undergone surgery to remove a blockage in his left carotid artery in October. Kennedy's office said that blockage was discovered "as part of a routine evaluation of Sen. Kennedy's back and spine."
He suffers chronic back pain from injuries suffered in a plane crash in 1964.
In recent days Kennedy has appeared to be in fine health. On Friday, he took part in a ribbon-cutting ceremony for a new maritime learning center in New Bedford.
Family members reported that Kennedy was well enough later in the morning Saturday to call to say he would not be able to join them for lunch.
Kennedy has represented Massachusetts in the U.S. Senate since 1962. He is one of only six senators in U.S. history to serve more than 40 years. He is known as a liberal champion of social issues such as health care, family leave and the minimum wage. Take a closer look at Kennedy's life »
Kennedy is the youngest of nine children in the famous family of Joseph and Rose Kennedy. His oldest brother, Joe, died in World War II; two other brothers, President John F. Kennedy and Sen. Robert Kennedy, were assassinated in 1963 and 1968, respectively.
CNN's Deb Feyerick contributed to this story.
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