Congress has invited U.S. President Donald Trump to its first impeachment hearing on 4 December.
If he does attend, the president would be able to question witnesses.
It would mark the next stage in the impeachment inquiry, which centres on a July phone call between Mr Trump and Ukrainian President Volodymyr Zelensky.
In that call, President Trump asked Mr Zelensky to investigate Joe Biden, currently the front runner to be the Democratic candidate in next year’s presidential election, and his son.
The probe is looking into whether Mr Trump used the threat of withholding US military aid to pressure Ukraine into investigating the Bidens.