The 38 were found guilty of links to violent election protests in 2005.
A US spokesman urged the Ethiopian government and the High Court to "promote much-needed reconciliation" in final sentencing.
The US is a close ally of Ethiopia's Prime Minister Meles Zenawi and helped his forces oust Islamists in Somalia.
Ethiopia's government has always stressed that the courts are independent and denies that the trial is political.
Relatives of the 38 Coalition for Unity and Democracy (CUD) leaders say they have signed a document which might pave the way for them to be freed.
The BBC's Elizabeth Blunt in Addis Ababa says this could be some form of apology or plea for mercy.
"We call on the Ethiopian government and High Court to take action in making a final sentencing determination, which is consistent with the greater objectives of bolstering the rule of law and promoting much-needed reconciliation," said US State Department spokesman Sean McCormack.
Among the 38 are the entire CUD leadership, several of the capital's elected MPs and city councillors, including Berhanu Negga, mayor-elect of Addis Ababa.
Prosecutor Abraham Tetemke said they had tried to bring down the government when he called for the death penalty on Monday.
Some 193 people were killed in protests at alleged vote-rigging.
Most of the dead were protesters, killed by security forces.
The judge had been due to pass sentence on Monday but he adjourned the hearing for a week to allow those convicted the chance to respond to the prosecutor's statement.
They have so far refused to recognise the court or enter any defence - the reason why the judges said they were forced to find them guilty as charged last month.
Their offences included outrage against the constitution and, in the case of party leader, Hailu Shawel, and four others, inciting, organising and leading armed rebellion.
Hundreds of thousands took part in demonstrations complaining of fraud and rigging in the elections won by Prime Minister Meles' party.
An independent inquiry carried out by an Ethiopian judge concluded that the police had used excessive force.
He went on to accuse them of carrying out a massacre. The judge later fled Ethiopia, saying he had been put under pressure to change his findings and had received death threats.
The government points out that it introduced multiparty elections to Ethiopia after years of military rule.
In the elections, the opposition made huge gains but says it was cheated out of victory.
Three months ago, a judge threw out controversial charges of attempted genocide and treason against another 111 people arrested after the election protests.
The violence and the charges of election fraud have tarnished Mr Meles' image as a favourite of Western donors and one of a new wave of reforming African leaders.Some donors have reduced aid over the case.