In Washington, old hands often say things like, "Never bet against Mitch McConnell." Well, the Kentucky Republican has some work to do.

McConnell is known as a deft behind-the-scenes tactician. Critics will say other than a 2017 Republican tax bill that passed with nary a Democratic vote, he has no major legislative victory over his long congressional career. But critics and allies alike acknowledge Mitch McConnell usually gets what Mitch McConnell wants.

So the language out of a Tuesday evening Senate Republican conference meeting was notable. Sources close to the majority leader noted he had informed his members he lacks the votes to block witnesses in the impeachment trial "at this time". With three full business days ahead before an expected Friday evening vote on the matter, Washington's ultimate inside player has time to lean into Republicans as he tries to bring the trial to an end.

House Democratic prosecutors and Mr Trump's defence team will be back on the Senate floor on Wednesday afternoon after the president's legal team closed their case on Tuesday by again not denying their client tried to use taxpayer-funded US military aid to get Ukraine's government to investigate his domestic political rivals. Rather, they attacked House Democrats' case, bashed the other party's process used to impeach Mr Trump in the House, questioned whether his "political sins" are even impeachable – while at the same time contending he did not one thing wrong.

The bipartisan comity in Washington as the Senate trial reaches a crescendo is almost non-existent. Mr Trump signed his NAFTA-replacing trade pact with Canada and Mexico on Wednesday morning. Despite it receiving ample Democratic votes in both chambers, he didn't invite a single member of the party that impeached him.

All the president's lawyers: The team fighting Trump's impeachment Show all 6 1 /6 All the president's lawyers: The team fighting Trump's impeachment All the president's lawyers: The team fighting Trump's impeachment Alan Dershowitz Dershowitz is a controversial American lawyer best known for the high-profile clients he has successfully defended. Those clients have included OJ Simpson, Jeffrey Epstein and Harvey Weinstein. One longtime Harvard Law associated told the New Yorker Dershowitz "revels in taking positions that ultimately are not just controversial but pretty close to indefensible." Getty All the president's lawyers: The team fighting Trump's impeachment Ken Starr Starr became a household name in the 1990s as the independent counsel who led the investigation that led to Bill Clinton's impeachment. That investigation began as a look into a real estate scandal known as Whitewater, and eventually led to impeachment after Mr Clinton lied under oath about having an affair with White House intern Monica Lewinsky. AP All the president's lawyers: The team fighting Trump's impeachment Jay Sekulow Sekulow is the president's longtime personal attorney, and, now, personal lawyer in the White House. He has been accused by former Rudy Giuliani associate Lev Parnas of being "in the loop" during the Ukraine scandal. Getty All the president's lawyers: The team fighting Trump's impeachment Pam Bondi Bondi is the former attorney general in Florida, and a longtime backer of the president's. She made a name for herself in Florida for taking hyper partisan stances on issues, and her penchant for publicity. She is likely to be a prominent public-facing figure during the trial. AFP/Getty All the president's lawyers: The team fighting Trump's impeachment Pat Cipollone Cipollone is the White House counsel, and leading the president's defence team. Getty All the president's lawyers: The team fighting Trump's impeachment Rudy Giuliani While not officially named as one of the president's impeachment lawyers, it is hard to ignore Giuliani's outsized role in this process. The former mayor of New York has been making headlines for months as he defends his client, and for his apparent role in the effort to compel Ukraine to launch the investigation into Joe Biden. We'll see how he figures in the actual trial, which he has said he would like to be a part of. Reuters

Here are three things to watch as the Untied States' third presidential impeachment trial heads into unchartered waters.

Whip in chief

Mr McConnell prefers to count votes and try to flip members to his side mostly in private.

But the leader of the Republican Party has a different approach, and it was on display on Wednesday morning. The president lit into John Bolton, the one-time White House national security adviser he fired and whose leaked account described the president's alleged desire to use the military aid as leverage, in the kind of tweet that GOP lawmakers fear.

Mr Trump described Mr Bolton, who could end up offering damning testimony in the Senate trial, as someone who "'begged' me for a non Senate approved job." He then said he made the longtime conservative hawk and Iraq war proponent a job as his top national security adviser "despite many saying 'Don't do it, sir'."

GOP lawmakers have mostly stood by the president despite his brash and sometimes personal attacks on friends and foes because they fear one presidential tweet lashing out at them could produce a primary challenger or the shutting off of the Republican Party campaign funding spigot Mr Trump controls.

So his tweet thread about Mr Bolton should be taken as a warning to any Republican senators thinking of breaking ranks and voting to bring in the former White House official in an impeachment trial that could now drag on for weeks or even months.

The commander in chief surely got Republican lawmakers' attention when he criticised his former subordinate for "many ... mistakes of judgement", saying he fired Mr Bolton "because frankly, if I listened to him, we would be in World War Six by now".

McConnell's move

Backed by Mr Trump's heavy hand, the majority leader needs to keep 51 of the chamber's GOP senators in line.

"The consensus is that we've heard enough and it's time to go to a final judgement," Wyoming Senator John Barrasso said on Tuesday, referring to his Republican mates.

Mr Barrasso is a member of Mr McConnell's leadership team, meaning his comments are a signal to Senate Republicans that the majority leader is ready to move to a still-expected vote – at some point – to acquit Trump on both impeachment articles passed by the House.

Rank-and-file GOP senators are publicly making the same case for a handful – or more – of their witness-curious colleagues.

See you in court?

Mr Trump said last week he would use executive privilege to block the testimony of a subpoenaed Mr Bolton, who says he would do so if presented with a legal summons. The president contends allowing a former senior adviser to testify before lawmakers would create a "national security problem".

There is a long history of a handful of top adviser-to-presidents positions being immune from appearing before Congress; the idea is to allow them to give commanders in chief their most candid advice in private.

In fact, House Intelligence Chairman Adam Schiff, the lead Democratic impeachment manager, said again this week that he opted against issuing Bolton a subpoena during his investigation to avert a long and bruising court fight.

If Mr McConnell cannot wrangle his caucus, Mr Trump will face a historic decision. So what happens if he moves to block Mr Bolton's testimony?

"The real answer is that no one knows. No president has attempted to use executive privilege to block testimony or evidence in a Senate impeachment trial. There are no Senate precedents dealing with executive privilege in impeachment," according to Jonathan Shaub, a former Justice Department official.