Top opposition Democrats say the US inquiry into alleged Russian meddling during the 2016 election could be under threat after President Donald Trump fired Attorney General Jeff Sessions.
Nancy Pelosi, who leads Democrats in the House of Representatives, called the decision a blatant attempt to end or impede the investigation.
The probe has been criticised by Mr. Sessions's successor Matthew Whitaker.
The Democrats, who won the House in the mid-terms, have vowed to protect it.
Some Republicans appear to have shared the concern of the Democrats over the future of the inquiry, which is being led by Special Counsel Robert Mueller.
Senator Susan Collins and Mitt Romney said it should not be impeded in any way.