It has been a week of explosive testimonies in public hearings for US President Donald Trump's impeachment inquiry.

On Tuesday, Army Lieutenant-Colonel Alexander Vindman testified, saying he heard a shocking phone call where President Trump made what he called "improper" demands of Ukrainian President Volodymyr Zelenskyy.

And on the same day, former Ukraine Special Envoy Kurt Volker told the hearing, he thought Trump's personal lawyer, Rudy Giuliani was feeding a negative narrative of Ukraine.

On Wednesday, EU Ambassador Gordon Sondland said he believed military aid had been withheld from Ukraine to pressure its government to investigate Trump's rivals.

Sam Berger, vice president of Democracy and Government Reform at the Center for American Progress, who had been a senior Domestic Policy advisor in the White House during Barack Obama's presidency, talks to Al Jazeera about the hearings.

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