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(CNN) President Donald Trump spent most of Monday defending his choice to stand down in Syria after a weekend spent on Twitter making claims about former Vice President Joe Biden, Democratic critics and the whistleblower whose report sparked the impeachment inquiry .

Here's the latest on the historic investigation:

The silent (Senate) majority

The main question, as Democrats move toward impeachment in the House, is whether the Republican-controlled Senate will close ranks around President Donald Trump -- or not.

CNN's Ted Barrett, Haley Byrd, Manu Raju, Sunlen Serfaty, Alex Rogers and Clare Foran have contacted more than 80 offices for Republican lawmakers in the House and Senate in recent days to ask for a response to Trump's remarkable public call for foreign governments to investigate former Vice President Joe Biden and his son Hunter.

From their report:

Only a few responded; just a handful have expressed any misgivings to CNN or other outlets.

... Why the silence? Trump holds powerful sway over congressional Republicans, largely because he is wildly popular with the same Republican voters who will decide whether to reelect these lawmakers. Most who defy Trump end up out of office, about the last place any politician wants to be.

A few symbolic objections

"The President should not have raised the Biden issue on that call, period. It's not appropriate for a president to engage a foreign government in an investigation of a political opponent," Sen. Rob Portman of Ohio told the Columbus Dispatch.

Along with a few other statements critical of Trump over the weekend and last week from Sens. Mitt Romney, Ben Sasse and Susan Collins, that's the beginnings of at least a small foundational crack in GOP support for Trump.

But there's a big difference between criticism and opposition. Indeed, Portman told the Dispatch of Trump's behavior, "I don't view it as an impeachable offense."

And he also said, "I think the House frankly rushed to impeachment assuming certain things," according to the paper.

Unrelated: Syria

It's not that Senate Republicans are afraid to disagree with Trump on everything.

Sen. Lindsey Graham of South Carolina has gone nearly apoplectic against Trump over Syria and called Trump's move to pull US troops and allow Turkey space to move in "shortsighted and irresponsible."

It was a surprising turnaround for a former Trump critic who's emerged as a key Senate ally, as well as a frequent golf partner. On the impeachment inquiry Graham has achieved status as presidential defender no. 1 against the whistleblower.

Also Monday, Senate Majority Leader Mitch McConnell said in a statement directed at Trump that there's a veto-proof majority in the Senate that disagrees with him.

But that willingness to stand up to Trump on Syria makes the failure to call Trump out over his public calls for foreign interference in US elections more jarring.

McConnell also has a new TV ad in which he paints himself as the one person who can save Trump's presidency.

Sondland testimony blocked

House investigators had hoped for multiple depositions this week, but they may only get one (behind closed doors).

US Ambassador to the EU Gordon Sondland did not appear on Tuesday after the State Department blocked his testimony.

Why he's important:

Key player in the nexus between Giuliani, Trump and the new Ukraine administration.

His defense of President Trump's intentions featured prominently in text messages released by House committees last week.

Former US Ambassador to the Ukraine Marie Yovanovitch is supposed to testify on Friday. After Sondland was kept from testifying, it will be interesting to see if Yovanovitch shows.

Why she's important:

Was recalled to the US before Trump's call with Ukrainian President Volodymyr Zelensky. In the call, Trump called her "bad news."

The former prosecutor in Ukraine, Yuriy Lutsenko, said she pressured him to stay out of US politics.

The Wall Street Journal reported Trump recalled her after complaints from Giuliani.

Who already wasn't talking to Democrats (yet)

Three key allies of Giuliani and two State Department officials likely won't testify this week before the impeachment inquiry. The committee has threatened subpoenas for Giuliani's associates.

Lev Parnas and Igor Fruman, associates of Giuliani

Why they're important:

They are Florida-based but were reportedly born in the then-Soviet Union

They gave hundreds of thousands in donations to a Trump-allied super PAC, according to the Miami Herald.

They have retained Trump's personal attorney John Dowd

They missed a deadline for documents today.

Parnas and Fruman are pursuing business opportunities for natural gas in Ukraine, according to an Associated Press investigation.

They've pushed for investigations of Burisma, the natural gas company that employed Hunter Biden.

Parnas was Giuliani's fixer in Ukraine, introducing him to current and former officials as far back as 2018, according to CNN's reporting.

Semyon "Sam" Kislin

Why he's important:

His website says he was on Giuliani's Council of Economic Advisers

House committees ask him to provide documents from after January 20, 2017.

A letter asked him to turn over documents by today and be deposed next week.

Ulrich Brechbuhl, Counselor of the State Department

West Point classmate, business parter and now key aide to Secretary of State Mike Pompeo.

Listed in the whistleblower complaint as a person who was possibly listening to July 25 Trump call with Ukrainian President Zelensky.

State has denied Brechbuhl was on the call. Turns out Pompeo was actually on the call.

Deputy Assistant Secretary of State George Kent

Why he's important:

Oversees US policy on Ukraine at the State Department.

Was a top official at the US embassy in Kiev until 2018.

Not mentioned in the whistleblower report.

Official communications on WhatsApp

From CNN's Kylie Atwood: Most of the text messages that Kurt Volker turned over to the congressional committees conducting the impeachment inquiry last week were carried out over WhatsApp, according to two sources. When Volker handed them over to the committees he simultaneously handed the same documents over to the State Department for record keeping purposes, explained one of the sources familiar with his actions. There are still some additional messages that Volker will hand over to State in the coming weeks, which are not relevant to the inquiry.

Federal record keeping requires all government employees to turn over official copies of any electronic messaging to the agencies they work for no later than 20 days after it was created. Volker did not turn over some of these messages in time to meet that deadline. The State Department did not reply to multiple questions from CNN.

How to keep the whistleblower anonymous

Democrats have to protect the identity of the whistleblower, who is in the unusual position of having been openly targeted by the President of the United States: CNN's Manu Raju and Zachary Cohen report on a list of ideas the Househave to protect the identity of the whistleblower, who is in the unusual position of having been openly targeted by the President of the United States:

The possibility of using an off-site location, limiting Hill staff and members who would be present and even disguising the individual's image and voice, the sources said.

Any secure facility, known as a Sensitive Compartmented Information Facility (SCIF), could be used for the meeting, potentially the facilities located at Langley or Fort Meade, secure facilities well outside downtown DC.

There are also ways to bring the whistleblower up to Capitol Hill without being seen by the press, including what one source described as the "Petraeus treatment" -- a reference to the former CIA Director and commander of US Central Command David Petraeus' private testimony from years back. Petraeus testified in 2012 as CIA director about the Benghazi embassy attacks.

Who are the 8 Democratic impeachment holdouts

What's below is from his report, which has maps and charts to drive these points home, but it is worth mentioning that these types of members are why Democrats have the majority:

Trump country

All eight come from districts Donald Trump won in 2016, and five of the eight are from districts Mitt Romney also carried in 2012.

Not only are they from Trump-won areas, their districts are among the top 15 largest margins of Trump victory of any seat in the county currently held by a Democrat. Trump won by double digits in all but two of the districts.

Slim margins

Five out of eight won in last November's midterms by fewer than two points.

Six out of eight are from districts that flipped from Republican to Democratic control in the midterms.

Demographics

Seven out of eight holdouts come from districts with fewer four-year college graduates than the national average.

Seven out of eight come from districts with a lower economic output, as measured by GDP, than the national average.

Six out of eight come from districts with proportionally whiter populations than the national average.

What else

Trump downsizes National Security Council -- The President has ordered a substantial reduction in the staff of the NSC, according to Bloomberg. It's not clear if the cuts come as a result of the whistleblower complaint. The President has ordered a substantial reduction in the staff of the NSC, according to Bloomberg. It's not clear if the cuts come as a result of the whistleblower complaint. LINK

Kurt Volker officially resigned from McCain Institute -- CNN had reported last week he was planning to leave ( CNN had reported last week he was planning to leave ( CNN

Perry speaks again -- Rick Perry said again Monday that he "asked the President multiple times" to call Zelensky. But Perry said he was encouraging talks about energy, not the Bidens. How does this help Trump since his ultimate talk with Zelensky ended up focused on investigations into the Bidens and the 2016 election? ( Rick Perry said again Monday that he "asked the President multiple times" to call Zelensky. But Perry said he was encouraging talks about energy, not the Bidens. How does this help Trump since his ultimate talk with Zelensky ended up focused on investigations into the Bidens and the 2016 election? ( CNN ).

Fact Checking Trump's picture of Biden

CNN's Daniel Dale fact checks all things Trump and he had several opportunities over the weekend. Here's one:

Trump tweeted Sunday of Biden: "Sleepy Joe said he never spoke to the Ukrainian company, and then the picture came out where he was playing golf with the company boss and Hunter ..."

Facts First: The photo Trump appears to be referencing does not show Joe Biden playing golf with the "company boss" of Burisma Holdings, the Ukrainian natural gas company where his son Hunter Biden was a board member. The person in the photo is an American named Devon Archer, a longtime business associate of Hunter Biden who has also served on the Burisma board. A board member is not the "company boss" -- nor an "exec," as The photo Trump appears to be referencing does not show Joe Biden playing golf with the "company boss" of Burisma Holdings, the Ukrainian natural gas company where his son Hunter Biden was a board member. The person in the photo is an American named Devon Archer, a longtime business associate of Hunter Biden who has also served on the Burisma board. A board member is not the "company boss" -- nor an "exec," as a previous Trump tweet had identified Archer.

The fourth person in the photo with Archer and the Bidens has not been publicly identified, but he is not Burisma owner Mykola Zlochevsky or top executive Taras Burdeinyi.

What are we doing here?

The President has invited foreign powers to interfere in the US presidential election.

Democrats want to impeach him for it.

It is a crossroads for the American system of government as the President tries to change what's acceptable for US politicians. This newsletter will focus on this consequential moment in US history. Let us know what you think by dropping us a line at impeachment@cnn.com.