An official investigation should be launched into three Democratic senators who threatened to withhold federal aid to Ukraine if the country's prosecutor general didn't assist with the Mueller investigation, said Sen. Rand Paul, R-Ky., on "Fox & Friends" Monday.

"The only thing consistent about the whole discussion of people threatening Ukrainian aid is that everybody seems to be doing it — on both sides," he said. "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 Mueller investigation, we may pull back on your aid.

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

Paul cited the 2018 letter during a Sunday interview with "Meet the Press" and identified the signators as Sens. Robert Menendez, D-N.J., Dick Durbin, D-Ill., and Patrick Leahy, D-Vt. He also lumped in Joe Biden's pressure of Ukranian prosecutors to back off an investigation into his son Hunter, who was paid $50,000 a month by the Ukrainian gas company Burisma Holdings, while it was under investigation for corruption.

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"If you're going to condemn Trump, you need to condemn the Democratic senators," he said Sunday. "It shouldn't just be one-sided."

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Paul said the investigation is nothing more than a partisan attack and predicted that any articles of impeachment passed by the House will be swiftly voted down by the GOP-controlled Senate.

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"It sounds like the Democrats have done exactly the same thing they're accusing the president of doing," he told Fox News.

"I think in the end it's going to be seen by the American public as very partisan. If the House sends it over I think it will be voted down in the Senate."