A significant part of the federal government is shut down indefinitely because President Trump is insisting on $5 billion for a border wall (or steel fence) and Democrats are saying no. But Trump's "wall" fixation actually began as little more than "a memory trick for an undisciplined candidate," Julie Hirschfeld Davis and Peter Baker report at The New York Times.

As Mr. Trump began exploring a presidential run in 2014, his political advisers landed on the idea of a border wall as a mnemonic device of sorts, a way to make sure their candidate — who hated reading from a script but loved boasting about himself and his talents as a builder — would remember to talk about getting tough on immigration. [The New York Times]

Trump embraced the idea once he saw the enthusiastic response his promise of a Mexico-funded border wall received among conservative audiences. "He's very obsessed about carrying out his campaign promises — I think to a degree that's unhealthy — but that's important to him, and that's not a bad thing," Trump friend Christopher Ruddy, CEO of Newsmax, tells the Times. White House counselor Kellyanne Conway says Trump is "focused on the wall" because "he thinks you need a very robust physical barrier at the border that you can't climb over, slide under, drive through or walk around." Her husband, George Conway, had a different interpretation:

Many of Trump's fellow immigration hardliners view his wall as a counterproductive, minor-to-insignificant part of broader restrictions on legal and illegal immigration. You can read more about the genesis of Trump's wall, and its critics on both sides of the immigration debate, at The New York Times. Peter Weber

Former New York City Mayor Rudy Giuliani claims he'd "love" to be a witness in President Trump's Senate impeachment trial, reports The Hill.

Giuliani, who serves as Trump's personal lawyer, told radio talk show host John Catsimatidis: "I would love to see a trial. I'd love to be a witness — because I'm a potential witness in the trial — and explain to everyone the corruption that I found in Ukraine, that far out-surpasses any that I've ever seen before, involving Joe Biden and a lot of other Democrats."

Giuliani was allegedly involved in a push to pressure Ukraine into launching investigations into Trump's political rivals, working to dig up dirt on former Vice President Joe Biden and pushing out former Ukrainian Ambassador Marie Yovanovitch, who was reportedly viewed as an obstacle in obtaining the promise of investigations.

Democrats in Congress have called for the Senate impeachment trial, set to begin arguments this week, to include additional witnesses beyond those who testified in the House inquiry. Senate Republicans have so far declined the requests. No evidence has emerged to prove any wrongdoing by Democrats in Ukraine, but Giuliani said "I have those facts. I have those witnesses. I have documents, and I have recordings. And I would love to get them out in public." Summer Meza

House Intelligence Committee Chair Adam Schiff (D-Calif.) on Sunday accused the National Security Agency of withholding key documents from Congress related to Ukraine that could be relevant in President Trump's impeachment trial.

Speaking to ABC News, Schiff said the NSA appears "to be succumbing to pressure from the administration," also saying "there are signs that the CIA may be on the same tragic course." Schiff said the documents could be important to the central allegation of Trump's impeachment: that he abused his power by withholding Ukraine military aid to push the country into announcing investigations of his political rivals.

The NSA and CIA have not commented on Schiff's allegations, reports NBC News.

Read more at NBC News and ABC News. Summer Meza

Fox News' Chris Wallace pointed out Sen. Lindsey Graham's (R-S.C.) updated view on witnesses in a Senate impeachment trial, but Graham swore the situation is now different.

House Democrats say "evidence overwhelmingly establishes" Trump's guilt ahead of his Senate impeachment trial, set to begin arguments on Tuesday. But they want to call new witnesses to testify, including former National Security Adviser John Bolton and Lev Parnas, an associate of Rudy Giuliani. Senate Republicans have so far denied the request.

Wallace said Graham's view that new witnesses should not appear "directly contradicts what you said as a Republican House impeachment manager in 1999 during the Clinton impeachment trial." At the time, Graham said "there may be some conflict that has to be resolved by presenting live witnesses. That's what happens every day in court and I think the Senate can stand that."

"Why were witnesses okay then, but they're over the line now?" asked Wallace.

Graham blamed the "railroad job" in the House, saying witnesses were available before the House voted to impeach Trump. "If they were that important, why didn't you call them in the House? Do you need them to make your case?" The Hill reports that in some cases, witnesses were not available or willing to testify until very recently. The White House also blocked several administration officials from appearing before the House. Summer Meza

House Democrats filed a 111-page legal brief ahead of President Trump's impeachment trial, arguing he threatens national security.

The House prosecutors laid out the argument against Trump that led to his impeachment last month on charges of abuse of power and obstruction of Congress. The legal brief says "evidence overwhelmingly establishes" Trump's guilt and says the Senate "must eliminate the threat" he poses.

The White House defense team, meanwhile, has not filed its official brief, but rejected the impeachment managers' arguments as "highly partisan." Without directly addressing allegations Trump abused his power by withholding Ukrainian aid to push for a politically-motivated investigation of his rivals, the White House castigated the "lawless process" that led to his impeachment.

Read more at The Washington Post. Summer Meza

Hong Kong protesters were hit with tear gas and pepper spray after demonstrators allegedly attacked a plainclothes police officer during a mass protest, reports The Washington Post.

Demonstrations have been ongoing for months, and have recently erupted in occasional violence as pro-democracy residents continue to protest the local government. Several protesters were arrested on Sunday after tens of thousands participated in a rally in Hong Kong's central district, the largest demonstration since New Year's Day when over a million people gathered.

A plainclothes officer reportedly refused to show a rally organizer his identification card, which led to an altercation. At least three people were injured. Read more at The Washington Post. Summer Meza

Former Vice President Joe Biden on Saturday claimed Sen. Bernie Sanders' (I-Vt.) campaign had released a "doctored" video that appeared to show Biden agreeing with Republican proposals regarding Social Security.

"It's simply a lie, that video is a lie," Biden told supporters of his presidential campaign in Iowa, per NBC News. He said "Bernie's people" had circulated the video, and he's "looking for his campaign to come forward and disown it, but they haven't done it yet." Politico reported the video "was not doctored by Sanders."

The video showed Biden agreeing with former House Speaker Paul Ryan's proposal to privatize Social Security, but Biden's campaign said he called the plan "correct" sarcastically, reports Bloomberg.

Sanders' campaign said Biden should "stop dodging questions about his record" and pointed to Biden's numerous other comments on the program. Read more at NBC News and Bloomberg. Summer Meza