Updated: 7:43 p.m.

Sen. Edward Kennedy (D-Mass.) was rushed to the hospital Tuesday after having a seizure during the inaugural luncheon at the Capitol, but doctors say the ailing Senator is already on the mend.

“After testing, we believe the incident was brought on by simple fatigue,” Dr. Edward Aulisi, chairman of neurosurgery at Washington Hospital Center, said in a statement Tuesday night.

“Sen. Kennedy is awake, talking with family and friends, and feeling well. He will remain at the Washington Hospital Center overnight for observation, and will be released in the morning,” Aulisi said.

The veteran Democrat began seizing during the lunch with newly inaugurated President Barack Obama. According to one source on the scene, “Somebody was holding him as he was seizing” until medics came and laid him out on a reclined wheelchair and rushed him out.

Sens. Orrin Hatch (R-Utah), John Kerry (D-Mass.) and Chris Dodd (D-Conn.) were among those who escorted Kennedy out of the luncheon and into an ambulance.

Kennedy didn’t say anything during the ordeal but at one point looked up and “kind of smiled that old Irish smile,” Hatch said. In that moment, Hatch said he felt that Kennedy was “going to be alright. But it was scary.”

Dodd said Kennedy was talking and responsive before he was taken to the hospital and said he was going to be OK. “The good news is he’s going to be fine,” Dodd said.

Kennedy spent the afternoon in the hospital with his wife and son, Rep. Patrick Kennedy (D-R.I.), according to a Washington Hospital Center spokeswoman. Obama, who was sworn in as president just hours earlier, also called Kennedy to check on his condition.

Before delivering his speech at the lunch, a somber Obama said his prayers were with Kennedy and his family. Kennedy, who was first diagnosed with a malignant brain tumor in May, was one of Obama’s most ardent supporters during the 2008 presidential campaign.

Sen. Jay Rockefeller (D-W.Va.) questioned whether Kennedy was up for handling the stress of the day. Kennedy has been a rare presence in the Senate in the months since his diagnosis.

“When you submit yourself to that kind of physical and emotional pressure, I’m not sure it’s a good idea,” Rockefeller said, adding that “maybe he was trying to do too much in one day.”

Former Vice President Walter Mondale, who sat with Kennedy at the luncheon, said Kennedy had been “in great spirits, telling us old war stories and then he seemed to have some kind of problem. … It was really kind of a shock to all of us.”

Mondale wouldn’t give details on what exactly happened to Kennedy at the table, but he said he was relieved to hear “he’s doing much better.”

Concerns were also raised about the health of Sen. Robert Byrd (D-W.Va.) during the presidential luncheon, but a spokeswoman for his office said Byrd did not fall ill after all.

Eyewitnesses said Byrd was escorted out of the luncheon just minutes before Kennedy was taken out and was trailed by a medical team.

But Byrd spokeswoman Jenny Thalheimer said the 91-year-old Byrd is in good health and that confusion arose when paramedics wheeled Byrd out of the way so that they had room to treat Kennedy.

“He got very upset when Sen. Kennedy got sick,” Thalheimer said of her boss. Byrd and Kennedy were seated at the same table in the Capitol Rotunda, she said, and Byrd later chose not to return to the table.

With the exit of four sitting Senators into Obama’s administration — including the president himself and Vice President Joseph Biden — Kennedy’s health issues and Byrd’s age threaten to further rob the Senate of valuable institutional knowledge and legislative experience.

Although Obama barely hung his hat in the chamber before ascending to the White House, and neither secretary of State nominee Hillary Rodham Clinton nor newly confirmed Interior Secretary Ken Salazar were longtime Senators, Biden served there for 36 years.

And between the two of them, Kennedy and Byrd constitute more than a century of service in the Senate.

Byrd’s role on Capitol Hill has diminished in recent years because of his age and declining health. But Kennedy is expected to play a leading role in shepherding Obama’s health care legislation through the Senate.

Should Byrd be unable to serve out the remainder of his term — which expires in 2012 — Gov. Joe Manchin (D) would appoint a successor. That individual would serve until the next regularly scheduled general election that falls two and a half years after the appointment is made.

Should Kennedy be unable to serve out the remainder of his term, which also expires in 2012, his successor would be chosen via special election only. The heavily Democratic Massachusetts Legislature removed the governor’s ability to appoint in 2004, out of fear that then-Gov. Mitt Romney (R) would use the power to name a Republican in the event of a vacancy.

Both states would likely see a bevy of candidates run for Senate in the event of an open seat or election featuring an appointed incumbent.

In West Virginia, Republicans would likely make a heavy push to recruit Rep. Shelley Moore Capito (W.Va.) to run. West Virginia Secretary of State Betty Ireland (R) is also seen a potential candidate and one who might be formidable.

State Democrats are hesitant to discuss potential Byrd successors out of deference to their beloved, nine-term senior Senator. However, Manchin is popular and is seen as the strongest possible candidate. Whether he would be willing to appoint himself to replace Byrd is unclear, although he could choose a caretaker who would not stand for election — and in doing so leave the door open for himself.

In Massachusetts, the all-Democratic House delegation has no shortage of individuals who look in the mirror and see a Senator: Reps. Ed Markey, Richard Neal, Bill Delahunt, John Tierney, Stephen Lynch, Barney Frank, Mike Capuano, Jim McGovern, John Olver and Niki Tsongas.

Former Rep. Marty Meehan (D), whose war chest is still flush with cash, would also have to be considered a potential Senate candidate in the event a seat opened up, as would former Rep. Joe Kennedy (D), the Senator’s nephew.

Hatch, for one, said he hopes Kennedy can come back to work soon. While the two of them are political opposites and may “fight each other all the time” on the Senate floor, they are like “fighting brothers.”

Kennedy is “one of the leading people on health care in the country, if not the leading person on their side,” the Utah Republican said. “Hopefully, we can … do what really needs to be done on health care and [Kennedy] knows he’s going to have to come to the center to get it done.”

John McArdle, Shira Toeplitz, Geof Koss and Stephen Langel contributed to this report.