The trial is not for the murder. It is for earlier "abuse of power" by arbitrarily freeing a jailed friend and supporter, General Lino Oviedo, last year. But it was pushed forward because of the killing and is certain to be influenced by it. Both houses of Congress publicly accused Mr Cubas of involvement as a result of an internal party split.
Political commentators said there appeared to be enough votes by Mr Argana's supporters and opposition senators to convict the President and oust him from office, possibly next week. He could also face criminal charges.
"I had another job before this one," the President told reporters in his office as the impeachment trial began. "I don't see what I should be nervous about."
Most Paraguayans disagreed. Many believe Mr Cubas and General Oviedo ordered the assassination of Mr Argana, 66, a longtime rival within the Colorado Party, to which all three belonged.
A split between the Cubas-Oviedo faction and Mr Argana's supporters had been an open secret since Mr Cubas took office in August, succeeding Juan Carlos Wasmosy.
Mr Wasmosy ordered General Oviedo's arrest after a failed coup attempt in 1996. He was jailed for 10 years but still tried to campaign, from behind bars, as Colorado Party presidential candidate last year until he was barred from the race. Mr Argana was also a candidate but it was Mr Cubas who was eventually nominated and who won the election.
On Tuesday Mr Argana's vehicle was riddled with bullets as he left for work in the capital, Asuncion, giving his bodyguards no time to react. So far there is no sign of the three gunmen, who wore military-style camouflage gear and acted with military-like precision.
Mr Argana, who served as foreign minister and as head of the Supreme Court under the dictator Alfredo Stroessner in the Eighties, was buried on Wednesday evening in the city's Recoleta cemetery as mourners waved Paraguayan flags and chanted that Mr Cubas should resign and that General Oviedo should be jailed.
Mr Cubas did not attend, after the Argana family warned that he would be thrown out. "Cubas is Oviedo's accomplice in this barbarous killing," said the family's lawyer, Jose Alberto Planas.
Confusion surrounded General Oviedo's situation last night. On Wednesday the government announced that he had given himself up to judicial authorities to return to jail if necessary. But he said later he had come forward only to "clarify" his judicial position and that he remained free.
The killing reverberated around South America, raising fears of a military takeover in Paraguay after 10 years of democracy.
Brazil reminded Paraguay that its membership of the regional Mercosur trade and customs bloc depended on it remaining a democracy.