The Senate is voting on the appointment of John Roberts as the next US Supreme Court chief justice.
If confirmed, Mr Roberts will be the youngest chief justice for 200 years
Mr Roberts is expected to be approved comfortably, with Republicans set to back him en masse after a judiciary panel endorsed him last week.
A Catholic with conservative views, Mr Roberts, 50, is President George W Bush's choice for the post.
He was nominated to replace long-serving Chief Justice William Rehnquist, who died earlier this month.
Mr Roberts will be sworn in at the White House immediately after his anticipated confirmation, ready to take his seat in time for a new Supreme Court session on Monday.
A former Harvard graduate and current appeals court judge, Mr Roberts will be the youngest chief justice in 200 years.
His youth could put him at the helm of the Supreme Court for a significant period of time, setting its tone for a generation.
Born in Buffalo, raised in Indiana
Graduated from Harvard in 1979
Was clerk to Chief Justice Rehnquist and then served in the Reagan administration
Serves on the District of Columbia appeals court
Has argued before the Supreme Court 39 times
A low-key, modest man, John Roberts will hold one of the most influential posts in US public life.
The justices of the Supreme Court have immense power and are appointed until they die, resign or are impeached.
They can overrule politicians and declare the decision of the president unconstitutional.
President Bush still has to nominate a candidate for a second position on the court, which has come about because of the retirement of Sandra Day O'Connor, who is seen as holding a key swing vote.
Democrats are expected to put up a stronger battle when her replacement is named.
The selection for the remaining vacant seat on the court will almost certainly affect its political balance and therefore be far more controversial, our correspondent says.