"So, this is the first time in history where a President has been impeached for a non-crime for events that never occurred", he said.
Both are believed to have first-hand knowledge of what Trump is accused of: using his powers illicitly to pressure Ukraine to help his 2020 reelection campaign by investigating his potential Democratic opponent Joe Biden.
Although Trump has denied any wrongdoing and called a phone call at the center of the impeachment case "perfect", House representatives last month approved articles of impeachment for abuse of power and obstructing Congress.
If no witnesses are called, it is hoped the impeachment trial can move to its final stages, with Trump keen to see it finished before he gives his State of the Union address on 4 February.
The effort derailed the career - and may have threatened the personal security of - a respected United States ambassador at the wishes of corrupt Ukrainian elements; used the promise of a White House meeting to extract the "deliverable" Trump wanted; and escalated to the suspension by Trump of security aid for Ukraine appropriated by Congress, the evidence indicates.
The previous two presidential impeachment trials, of President Clinton in 1999 and President Andrew Johnson in 1868, resulted in acquittals.
The dueling Democratic and Republican narratives heading into the Senate trial reflect the high political stakes even though Trump's acquittal is nearly guaranteed. Ted Cruz told Fox News' "Sunday Morning Futures" that the trial could extend "to six to eight weeks or even longer" if the Senate made a decision to hear from additional witnesses - a prospect that could interfere with the imminent presidential primary contests, as Sens. "[My responsibility is] not to focus on the politics of where we are, but a recognition that we are in the midst of an infrequent and in many ways extraordinary process that the Constitution allows for, and I'm going to take my constitutional obligations very, very seriously".
Trump said that he "acted with extraordinary and unprecedented transparency" by releasing the transcripts of both calls that triggered the impeachment probe in the first place, while calling Democratic subpoenas "unconstitutional" and noting that his aids "are absolutely immune from completed testimony" in line with bipartisan practice of previous administrations.
Trump is publicly, at least, showing little concern as he thinks he knows what is going to happen.
"They are not strict legal arguments and they will not stand the arguments on the floor", he added.
In a statement Thursday, Collins said she hadn't decided yet.
If four Republicans do cross over, that could add weeks to the trial - three in the Clinton case. Joe Biden, in 2018, appeared to boast that he forced the firing of Ukraine's top investigator, purportedly to thwart a probe of the company tied to his son, by withholding from Ukraine $1 billion in USA loan guarantees. "Fox News Sunday" anchor Chris Wallace did his best to get some of those answers out of Democratic Caucus Chair Hakeem Jeffries, one of the seven impeachment managers who Pelosi designated for the trial. Republicans say that is proof that Trump did not engage in a reciprocal quid pro quo deal - the military aid in exchange for the investigations to help him politically.
"I'd like to personally hear from everyone hear from Hunter Biden".
Rep. Hakeem Jeffries, one of the 10 House impeachment managers, said it would be up to the Senate, but clearly wasn't thrilled with the idea.