CHICAGO (Reuters) - The former chief of staff for ousted Illinois Governor Rod Blagojevich pleaded guilty on Wednesday and agreed to testify about attempts to cash in on the governor's power to fill a vacant U.S. Senate post.

John Harris, 47, became the first of six defendants indicted in the corruption case against Blagojevich to reach a plea agreement with prosecutors. A trial is scheduled for June.

Prosecutors have said Blagojevich was arrested in December to prevent him from carrying out tape-recorded threats to trade the Senate seat vacated by newly elected President Barack Obama. Two months later, Blagojevich was impeached and thrown out of office by the state legislature.

"Working for Governor Blagojevich was an extraordinarily difficult thing to do ... In many situations John tried to prevent Governor Blagojevich from doing things which were illegal," Harris' attorney, Terry Ekl, said after Judge James Zagel accepted Harris' guilty plea to one count of wire fraud.

But the second-term Democratic governor ignored Harris' advice and sought to enrich himself by attempting to barter the senate seat for campaign cash, high-level jobs in the Obama administration, ambassadorships or other positions for himself or his wife, according to the 19-count indictment handed up in April.

In the plea agreement read by prosecutors in court, Blagojevich ordered Harris to threaten contractors to pony up campaign contributions and get newspaper editorial writers critical of him fired. In some cases, Harris did not carry out his boss' orders.

Blagojevich has denied wrongdoing and pleaded not guilty.

Ekl said Harris, who worked for Chicago Mayor Richard Daley before joining Blagojevich's staff in 2005, had been asked about other subjects during his six months of cooperating with prosecutors. Ekl would not be more specific.

U.S. Attorney Patrick Fitzgerald has won numerous convictions of functionaries and aldermen in Chicago's city government. Daley has been questioned by prosecutors but never implicated in the case.

Zagel told Harris he was likely to be sentenced to no more than 35 months in prison.

(Reporting by Andrew Stern; Editing by Cynthia Osterman)