"I hope I will be proven wrong, but it seems that impeachment based on partisan differences seems to be becoming the norm, not the exception", Graham said on Twitter on Saturday, adding "I fear that if this model is followed in the future – impeachment to disqualify one from holding office based on partisan hatred – will become the norm".
Earlier on Saturday, 57 US senators endorsed and 43 rejected a single article of impeachment accusing former US President Donald Trump of inciting the 6 January attack on the Capitol. A two-thirds majority vote or at least 67 of 100 Senators was needed for a conviction.
US President Joe Biden said after Trump was acquitted that "the substance of the charge is not in dispute", that "democracy is fragile" and "must always be defended" and "that is how we end this uncivil war and heal the very soul of our nation".
Senate Minority Leader Mitch McConnell said after Saturday's vote Trump's actions that preceded the riot were "a disgraceful dereliction of duty", and that the former president "is practically and morally responsible for provoking the events of the day [6 January]". Nonetheless, McConnell voted to acquit Trump on Saturday.
In a statement after the vote, Trump said the impeachment trial was "another phase of the greatest witch hunt in the history of our country".
Trump is the first president in US history to be impeached twice. The Republican-controlled Senate cleared him last February of charges of pressuring Ukraine to probe Joe Biden and his family.