With several Democrats openly floating the possibility they might vote to acquit President Trump, congressional Republicans are planning an aggressive "Plan B" strategy in the event some Republicans break off and demand additional witnesses in the president's impeachment trial, Fox News has learned.

Senate Majority Leader Mitch McConnell, R-Ky., privately said early Tuesday that he wasn't sure there were enough Republican votes to block more witnesses, given that some moderates in the GOP's 53-47 Senate majority were wavering. Any witness resolution would likely require four Republican defections in the Senate, because in the event of a 50-50 tie, Chief Justice of the United States John Roberts is highly likely to abstain rather than assert his debatable power to cast a tiebreaking vote.

Late Tuesday night, a Senate leadership source told Fox News that Republicans were specifically assessing the viability of two alternative options.

One plan is to amend any resolution calling for a particular witness to also include a package of witnesses that assuredly wouldn't win enough support in the Senate. For example, if the Democrats seek to call former National Security Advisor John Bolton, Republicans might subpoena Hunter Biden over his lucrative board position in Ukraine, and Rep. Adam Schiff, D-Calif., over his inconsistent statements concerning his panel's contacts with the whistleblower at the center of the impeachment probe.

"After listening to the Dems’ 20+ hours of argument and the rebuttal arguments from @realDonaldTrump, I’ve got lots of questions for the Dems," tweeted Missouri GOP Sen. Josh Hawley on Tuesday. "Like this one: Why did Schiff lie about his contact with the 'whistleblower'? More to come!"

The "package deal" proposal could afford moderate Republicans the political cover of supporting more witnesses in theory, while ultimately rejecting a witness package they deem flawed. Even if a witness package passed, the resolution could be written such that the witness phase of the trial ends immediately if a key witness, such as Hunter Biden, defies his subpoena.

WHOOPS: BIDEN CAMP TOUTS UKRAINE ACTIVIST WHO CALLED HUNTER BIDEN'S ACTIONS 'VERY BAD'

The Federalist senior editor and Fox News contributor Mollie Hemingway observed that in the House impeachment proceedings, Democrats allowed themselves to call significantly more witnesses than they afforded to their Republican colleagues -- raising the possibility Republicans could reasonably insist on an equally favorable "ratio" in the Senate trial when putting together a prospective package of witnesses.

A lopsided ratio in favor of the GOP also could lead some Democrats to oppose a witness package.

Another option, the congressional leadership source told Fox News, is for the White House to assert executive privilege to block witnesses, including Bolton. The administration could also head to court to obtain an emergency injunction against his testimony and the release of his new manuscript by citing national security concerns about the disclosure of highly classified information.

Federal criminal law prevents the disclosure of classified information, and the broader judicial doctrine of executive privilege separately has long shielded executive branch deliberations from disclosure.

The Trump administration's possible efforts might end up in a court battle, and could prove dicey if Bolton opts to go rogue and defy the White House's assertion of privilege -- or its concerns over classified information -- as the issues make their way through the courts.

Meanwhile, Politico reported on Tuesday that Democrats were apparently divided over whether to remove Trump from office on the charges of obstruction of Congress and abuse of power -- neither of which is a defined federal crime. Moderate Democratic Sens. Joe Manchin, Doug Jones, and Kyrsten Sinema were all weighing votes to acquit Trump on at least one of the two articles of impeachment, the outlet reported.

“I know it’s hard to believe that. But I really am [undecided]. But I have not made a final decision. Every day, I hear something, I think ‘this is compelling, that’s compelling,’” Manchin said. “Everyone’s struggling a little bit.”

TRUMP HOSTS MASSIVE RALLY IN NEW JERSEY, AS CROWDS LINE MOTORCADE ROUTE

That news came shortly after a disputed Los Angeles Times report that California Sen. Dianne Feinstein was considering a vote to acquit the president. Feinstein later said she was "misunderstood."

GOP senators were similarly all over the map on Tuesday as Trump’s defense team called Bolton’s new manuscript “inadmissible” and warned against opening the door to new wild-card information in the ongoing trial.

Trump told Bolton in August, according to an excerpt of Bolton's forthcoming book reviewed by The New York Times Times, "that he wanted to continue freezing $391 million in security assistance to Ukraine until officials there helped with investigations into Democrats including the Bidens." Republicans and liberal law professor Alan Dershowitz have countered that, even if true, the allegations do not rise to the level of an impeachable offense.

The White House has also argued that Bolton may have leaked the manuscript to improve sales of his book, which went live for pre-orders on Amazon just hours after the Times report broke.

"Why didn’t John Bolton complain about this “nonsense” a long time ago, when he was very publicly terminated," Trump tweeted late Tuesday. "He said, not that it matters, NOTHING!"

And, Republicans have reiterated, it would be legitimate for Trump to probe the Bidens' possible corruption for public policy reasons, given that Joe Biden openly boasted about successfully removing the Ukrainian prosecutor investigating the company where his son Hunter obtained a lucrative board position with no relevant experience. Biden was overseeing Ukraine policy when his son got the job at the Ukrainian company, which raised red flags at the time in the Obama administration.

Nevertheless, several Republicans have indicated they would be interested in hearing what Bolton has to say, at least in some capacity, and they left the door open on Hunter Biden.

'COUP HAS STARTED,' UKRAINE WHISTLEBLOWER'S ATTORNEY PROMISED IN 2017, PROMISING TO IMPEACH AND 'GET RID OF' TRUMP

Louisiana GOP Sen. Bill Cassidy, for his part, raised questions even as he denied widespread reports saying that he wanted to call more witnesses. Cassidy insisted that he wanted to wait until the end of the written question period of the trial to decide on witnesses.

That less-than-strenuous denial led Sean Davis, the co-founder of The Federalist, to argue that Cassidy was issuing a "Romney-esque non-denial," especially for a Republican in a deep-red, pro-Trump state.

Meanwhile, Sen. James Lankford, R-Okla., called for Bolton’s unpublished manuscript to be made available for senators to read in a classified Sensitive Compartmented Information Facility (SCIF) to understand what Bolton was alleging. His proposal got an ally in influential Sen. Lindsey Graham, R-S.C., who called the idea a “reasonable solution.”

Some senators suggested that Bolton just spill the beans in a news conference on the sidelines of the impeachment trial -- a proposal that could lead to legal questions concerning both executive privilege and classified information.

Bolton's manuscript is currently in a "pre-publication review" at the National Security Council, which functions as the White House's national security forum. Such a review is standard for any former government official who held security clearances and publicly writes or speaks publicly about their official work.

The review focuses on ferreting out any classified or sensitive material in advance of publication and could take from days to months.

WHAT'S THIS? LEV PARNAS WANTS TO SHOW UP AT THE IMPEACHMENT TRIAL, BUT HE HAS AN ANKLE MONITOR ON?

“The Wall Street Journal has called for John to just come forward -- just tell the public what you know,” Sen. Ron Johnson, R-Wis., said. “I think that actually [would] be a smart thing. I’d encourage John to do that without involving the trial.”

Separately, Harvard Law School Professor Alan Dershowitz, who delivered a spirited constitutional defense of the president on Monday night in the impeachment trial, took aim at Elizabeth Warren after she said she couldn't follow Dershowitz's argument.

"He is a criminal law professor who stood in the well of the Senate and talked about how law never inquires into intent and that we should not be using the president's intent as part of understanding impeachment," Warren said Monday. "Criminal law is all about intent. Mens rea is the heart of criminal law. That's the very basis of it. So it makes his whole presentation just nonsensical. I truly could not follow it."

Dershowitz replied on Twitter that Warren, who formerly taught at Harvard Law School, "doesn't understand the law" and had "willfully mischaracterized" his argument.

“If Warren knew anything about criminal law she would understand the distinction between motives – which are not elements of crime—and intent, which is. It’s the responsibility of presidential candidates to have a better understanding of the law,” Dershowitz said.

On Monday, flatly turned toward House impeachment managers and declared they had picked "dangerous" and "wrong" charges against the president -- noting that neither "abuse of power" nor "obstruction of Congress" was a crime, or even remotely close to an impeachable offense as the framers had intended.

In a dramatic primetime moment, the liberal constitutional law scholar reiterated that although he voted for Hillary Clinton, he could not find constitutional justification for the impeachment of a president for non-criminal conduct, or conduct that was not at least "akin" to defined criminal conduct.

WATCH: DERSHOWITZ CALLS OUT HOUSE DEMS IN SPIRITED CONSTITUTIONAL DEFENSE AT IMPEACHMENT TRIAL

"I'm sorry, House managers, you just picked the wrong criteria. You picked the most dangerous possible criteria to serve as a precedent for how we supervise and oversee future presidents," Dershowitz told the House Democrats, including head House impeachment manager Rep. Adam Schiff, D-Calif.

He said that "all future presidents who serve with opposing legislative majorities" now face the "realistic threat" of enduring "vague charges of abuse or obstruction," and added that a "long list" of presidents have previously been accused of "abuse of power" in various contexts without being formally impeached.

The list included George Washington, who refused to turn over documents related to the Jay Treaty; John Adams, who signed and enforced the so-called "Alien and Sedition Acts"; Thomas Jefferson, who flat-out purchased Louisiana without any kind of congressional authorization whatosever; John Tyler, who notoriously used and abused the veto power; James Polk, who allegedly disregarded the Constitution and usurped the role of Congress; and Abraham Lincoln, who suspended habeas corpus during the Civil War. Ronald Reagan, George H.W. Bush, and others would also probably face impeachment using the Democrats' rules, Dershowitz said.

"Abuse of power," he argued, has been a "promiscuously deployed" and "vague" term throughout history. It should remain a merely "political weapon" fit for "campaign rhetoric," Dershowitz said, as it has no standard definition nor meaningful constitutional relevance.

Dershowitz then said he was "nonpartisan" in his application of the Constitution, and would make the same arguments against such an "unconstitutional impeachment" if Hillary Clinton were on trial -- passing what he called the "shoe on the other foot" test.

He asserted that it's extraordinarily difficult to fairly determine that a president "abused" his power with an improper personal motive, given that national public policy objectives and personal political gain so often overlap. Hypothetically, if a President Bernie Sanders appeased his supporters by withholding aid to Israel unless it stopped building in contested settlements, it could be argued that he was obtaining some personal political benefit through a "quid pro quo" by doing so -- but his impeachment would be absurd, Dershowitz's argument suggested.

Similarly, it could plausibly be argued Trump was pursuing public policy goals when he sought to determine whether the former vice president was corruptly influencing Ukraine policy to benefit his son. Peering into the mind of the president to determine a true motive is not the job of a Senate, Dershowitz said, and instead resembles the behavior of a parliament.

Trump's lawyers wrapped up their opening arguments early on Tuesday. Starting on Wednesday, Democrats and Republicans will alternate in posing their questions to the House Democratic impeachment managers and Trump's legal team. Questions will be in writing, submitted to Roberts and read aloud. Senators do not pose the questions themselves. They must sign the questions, which may come from a group of senators or an individual senator.

Fox News is told to expect between 10 and 12 questions per side before a recess. There is no time clock as to how long counsel for both sides has to respond, but Roberts said Tuesday that based on the 1999 impeachment trial precedent, both sides should try to limit their responses to five minutes.

At the same time, Roberts noted according to the Congressional Record from 1999, everyone laughed at that suggestion. Senators laughed on the floor again Tuesday when Roberts hinted at the unofficial time restriction. There can be no challenge of given answers by counsel for either side.

CLICK HERE TO GET THE FOX NEWS APP

After written questions are over, the Senate will consider whether to hear additional documents and evidence. A final vote on the two articles of impeachment will follow, with a highly improbable two-thirds vote needed to convict and remove the president.

If, as expected, the Senate does not meet that threshold, Trump will have been formally acquitted.

Fox News' Marisa Schultz, Chad Pergram, and Fox Business' Hillary Vaughn, contributed to this report.