​The criminal trial of two top aides of former Ontario premier Dalton McGuinty will continue, after a judge in Toronto ruled Thursday the Crown has presented enough evidence of wrongdoing that the defence must respond.

However, a judge downgraded one of the charges against David Livingston, who was McGuinty's chief of staff during his final months in power, and his deputy, Laura Miller.

Lawyers for the pair had asked Justice Timothy Lipson to dismiss all charges. They were seeking a directed verdict of acquittal, arguing no evidence of a crime had been presented in court during three weeks of testimony by Crown witnesses.

The judge reduced the charges of committing mischief to data, and said the pair will instead be tried on a charge attempting to commit mischief to data. They are also charged with unauthorized use of a computer.

The charges against Livingston and Miller were laid over allegations the two senior political advisers destroyed government documents by ordering the wiping of 20 computers in the premier's office, allegedly in an attempt to hide information about the cancellations of two gas-fired power plants.

The cancellations of the gas plants in Mississauga and Oakville cost the province some $1.1 billion, according to Ontario's auditor general. Opposition pressure for the government to release documents about the decisions were at their height in late 2012 when McGuinty announced his resignation.

David Livingston, who was chief of staff to former Ontario premier Dalton McGuinty in 2012 and 2013, is also facing charges. (CBC)

In his ruling Thursday, the judge said the evidence presented so far could allow for a conclusion that Livingston "dishonestly obtained" special administrative access "under false pretences" to arrange for the computers to be wiped.

However, the judge stressed that his testing of the evidence at this stage did not involve weighing it to determine an actual verdict of guilty or not guilty.

Lipson said that while there was evidence that the pair attempted to delete data from computers without the proper authority, the Crown failed to present evidence any relevant data was actually wiped. That's why he ordered the downgrade of the mischief charge.

Lipson has presided alone over the Ontario Court of Justice trial at Toronto's Old City Hall courthouse. The case is to continue next Thursday when defence lawyers are to indicate how they will proceed. The defence could call witnesses or it could choose to proceed directly to final arguments.

In court earlier this week, Crown prosecutor Tom Lemon said he was aiming to prove that Livingston and Miller did not have the proper authority to erase data from the hard drives.

Prosecutors allege Peter Faist was hired to wipe clean about 20 hard drives in McGuinty's office in 2013. (Ved Lesik/CBC)

Previous testimony indicated that Miller's spouse, Peter Faist, an IT consultant who did not work in the Ontario public service, was given a special administrative password and paid nearly $10,000 by the Liberal caucus to erase hard drives on the government computers. That was in early 2013, just before McGuinty handed over power to Premier Kathleen Wynne.

The defence argued the hard drives were wiped to remove information that did not need to be retained from the computers of staff who were leaving the premier's office.

Last week, the Crown dropped breach of trust charges against the pair, admitting it had no reasonable chance of getting a conviction.

This is the second trial involving allegations of political corruption against Ontario Liberals to take place this fall.

Last month, a judge threw out all bribery charges against Wynne's former deputy chief of staff, Pat Sorbara, and a local party organizer, Gerry Lougheed Jr. The pair were declared not guilty before the defence had to present its case.