The bill, which will face a vote in the House of Commons on Wednesday, has provoked outrage from civil liberties groups and risks a revolt by Labour members that could hand Prime Minister Gordon Brown his first parliamentary defeat.
"This is about doing the right thing for the country," Smith told BBC television's Andrew Marr Show. "That is why I am asking people to support this on Wednesday."
It was the latest attempt by Brown's ministers and government managers to push through a change in legislation that they argue is crucial to combat a growing terrorist threat.
Defeat for Brown could critically damage his already waning authority after the loss of a formerly safe parliamentary seat, a near wipe-out in local elections, a sharp slowdown in the economy and an embarrassing U-turn on tax reform.
An ICM poll in the Sunday Telegraph showed Labour 16 points behind the main opposition Conservatives on 42 percent and only five points ahead of the third-placed Liberal Democrats on 21.
It said it was the lowest rating recorded for Labour in any ICM poll. Another poll last week said Brown's approval rating was the lowest for a Labour leader since World War Two.
But the same poll offered Brown a glimmer of hope, showing that 65 percent of people questioned supported the detention extension to 42 days and only 30 percent said it should stay as it is -- the position taken by the Conservatives.
