Prosecutors said a senator charged in a huge graft investigation centered on state oil company Petrobras had accused Rousseff of sending a powerful cabinet minister to try to buy his silence.

The senator, Delcidio Amaral of the ruling Workers' Party (PT), told investigators in statements given as part of a plea bargain that Education Minister Aloizio Mercadante, Rousseff's former chief of staff, contacted him via an aide to urge him not to testify.

Amaral, the former party leader in the Senate, said the minister indicated the message came from Rousseff herself – an accusation the leftist leader angrily denied.

The President "vehemently and indignantly repudiates the attempt to associate her name with the personal initiative of Minister Aloizio Mercadante," her office said in a statement.

As the long-running scandal swirled, administration sources said former president Luiz Inacio Lula da Silva – who is himself facing charges –  was on the verge of taking a cabinet post in Rousseff's administration to try to save the crisis-hit government.

Rousseff is facing an impeachment drive, a deep recession and mass protests, while her mentor's legacy is threatened not only by the charges against him but also by perceptions that his entire administration was underpinned by graft.

Lula arrived in Brasilia late Tuesday, where he met with the President for more than four hours to discuss options. Talks were expected to continue today, O Globo said on its website.

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