Washington ? Seizing a historic opportunity to reshape the Supreme Court, President Bush swiftly chose conservative John Roberts as chief justice Monday and weighed how to fill another vacancy that could push the nation’s highest court to the right on issues from abortion to affirmative action.

Polished and plainspoken, Roberts had been on a likely track to be confirmed as an associate justice and it appeared Bush turned to him for the top job to avoid an acrimonious fight at a volatile moment. Bush was on the defensive about the administration’s sluggish response to Hurricane Katrina and his poll ratings had fallen to their lowest point of his presidency.

“For the past two months members of the United States Senate and the American people have learned about the career and character of Judge Roberts,” Bush said. “They like what they see.”

Roberts’ nomination, just two days after the death of Chief Justice William H. Rehnquist, raised fears among Democrats about a rightward shift as Bush fills two openings on the nine-member court. Democrats have been frustrated by Roberts’ popularity and said the Senate must take a closer look at his nomination.

“The stakes are higher and the Senate’s advice and consent responsibility is even more important,” said Senate Minority Leader Harry Reid, D-Nev. Sen. Edward M. Kennedy, D-Mass., expressed concerns about the court’s balance.

“Replacing two justices at the same time will have an enormous impact on the court and on the lives and liberties of all Americans for decades,” said Ralph Neas, president of the liberal advocacy group People for the American Way, which opposes Roberts’ nomination.

Supreme Court nominee John Roberts, left, speaks as President Bush looks on in the Oval Office of the White House Monday in Washington. President Bush on Monday nominated John Roberts to succeed William H. Rehnquist as chief justice, and called on the Senate to confirm him before the Supreme Court opens its fall term on Oct. 3.

Senate Majority Leader Bill Frist, R-Tenn., said Roberts was “one of the most well qualified candidates to come before the Senate.” He said he still expects Roberts to be confirmed before the new court session begins Oct. 3.

Like Rehnquist, Roberts is deeply conservative. He was nominated in July to succeed retiring Justice Sandra Day O’Connor, who angered conservatives with her tie-breaking votes on contentious issues like abortion restrictions, campaign finance limits, discrimination laws, and religion.

The Roberts-for-Rehnquist nomination would not affect the balance, but Bush could force an ideological shift by replacing O’Connor with a reliably conservative vote. O’Connor has offered to remain on the bench until her successor is named, and Bush called her Monday to say he would move quickly to find her replacement as well. He is not expected to name a new O’Connor successor this week.

The change in Roberts’ nomination scrambled plans for his confirmation hearings, which had been scheduled to begin today. Instead, the capital prepared for the ritual of honoring Rehnquist. His body will lie in repose in the Great Hall of the Supreme Court today and Wednesday and he will be buried at Arlington National Cemetery following funeral services Wednesday.

Moving swiftly, Bush met with Roberts in the private residence of the White House for about 35 to 40 minutes on Sunday evening, then officially offered him the job at 7:15 a.m. Monday when Roberts arrived at the Oval Office.

Only 50 years old, Roberts could be a conservative legal force for decades – as was Rehnquist, who served 33 years on the court, 19 of them as its leader.