Democrats must be probed over Ukraine issue: US senator

Republican Senator Rand Paul said an investigation should be launched into three Democratic senators for threatening to withhold aid to Ukraine if the country's prosecutor general didn't collaborate with the Mueller probe.

"The only thing consistent about the whole discussion of people threatening Ukrainian aid is that everybody seems to be doing it -- on both sides," Paul told Monday during an interview with Fox News.

"Joe Biden threatened the aid and then three senators wrote a letter to the Ukrainian government, and they said if you don't continue investigating Trump, and help the [Robert] Mueller investigation, we may pull back on your aid," he said, referring to the ongoing probe into Russia's alleged interference in the 2016 White House race and President Donald Trump's campaign.

"Democrats get to threaten Ukrainian aid and nothing happens to them. Now they want to impeach a president over this?" Paul asked and added: "We can't have two standards."

On Sunday, Paul called the impeachment investigation on Trump "nothing more than a partisan attack," and added any articles of impeachment passed by the House will be swiftly voted down by the Republican-controlled Senate.

"If you're going to condemn Trump, you need to condemn the Democratic senators," he said, referring to Sens. Robert Menendez, Dick Durbin, and Patrick Leahy.

Trump is currently facing a formal House impeachment inquiry on allegations he used his office for political gains by pressing foreign leaders to investigate a political rival.

The inquiry centered on a July 25 telephone call in which Trump pressed Ukrainian President Volodymyr Zelensky to investigate Joe Biden, and his son, Hunter over corruption allegations.

Paul's comments came on same day Fiona Hill, a former top National Security Council expert on Russia, was testifying to Congress behind closed doors.

She is the first White House official to appear as part of the House impeachment inquiry.