WASHINGTON — Chief Justice John Roberts swore in Barack Obama as president of the United States on Tuesday with a slight stumble over the wording of the oath of office, a stumble Obama repeated.

Separated by a Bible used by Abraham Lincoln at his first inaugural, Roberts asked Obama: "Are you prepared to take the oath, Senator?"

The former Illinois senator indicated he was, and Roberts started reciting — and Obama repeating — the 35-word oath that is prescribed by the Constitution.

But at one point, Obama paused abruptly after Roberts reversed several words in the oath.

The oath includes the phrase "that I will faithfully execute the office of president of the United States" but Roberts didn't say "faithfully" until after saying "president of the United States."

Obama apparently realized that something was out of order.

Roberts then repeated the phrase correctly, putting "faithfully" in the right place after execute: "faithfully the office of president of the United States."

But Obama instead repeated Roberts' original, incorrect version: "... the office of president of the United States faithfully."

Later, as they shook hands before a lunch in the Capitol, Roberts chatted briefly with Obama and appeared to say that the mistake was his fault, not Obama's.

Despite the brief awkward moment, the swearing in was the first of what could be many important interactions between two men who rose to their positions of power quickly and who have some background similarities, but whose politics differ.

The affable Roberts and his conservative-leaning Supreme Court could have much to say in the years to come about Obama's most important policy choices.

Former President George W. Bush left the court with two relatively young and reliably conservative voices, those of Roberts, 53, and Justice Samuel Alito, 58. Roberts took his seat in 2005 and Alito joined him the next year.

Roberts is the youngest chief justice in more than 200 years. He easily could still be in his role a quarter century from now, long after Obama has left office.

He and Obama are similar in many ways. Both are late baby boomers. Roberts is 53, Obama 47. And both got their law degrees from Harvard and made rapid ascents to power. But their politics diverge sharply.

Roberts was an official in Republican administrations before becoming an appeals court judge and then chief justice under Bush.

Obama was one of 22 Senate Democrats to vote against Roberts' confirmation to the Supreme Court in 2005 — the first time a Supreme Court justice has sworn in a president who voted against him.

As president, Obama will try to use Supreme Court vacancies to counter Roberts' influence, either by replacing aging liberals with justices as young as or younger than Roberts or by changing the court's balance if a conservative justice retires unexpectedly.

Roberts added the words "so help me God" to the end of the constitutional oath, following a practice established by George Washington and followed by most presidents. And Obama repeated the phrase.

The last time a chief justice swore in a president of a different party was in 1997, when Chief Justice William H. Rehnquist, a Republican, swore in Democrat Bill Clinton for a second term. Two years later, Rehnquist would preside over Clinton's impeachment trial in the Senate which resulted in an acquittal.

Obama didn't actually finish taking the oath until 12:05 p.m., five minutes after he actually became president under the Constitution. Clinton, in his first inauguration in 2001, also was five minutes late in taking the oath.

The Lincoln Bible used by Obama was on loan from the Library of Congress.