First Read is your briefing from Meet the Press and the NBC Political Unit on the day's most important political stories and why they matter.

WASHINGTON — The country woke up Monday morning to news of horrific violence in Las Vegas, where at least 50 people were killed and 400 were injured at a country music concert, making it the deadliest mass shooting in U.S. history.

Per NBC’s Pete Williams, law-enforcement officials identified the gunman as 64-year-old Stephen Paddock from Mesquite, N.V., and he was shot dead by police.

As everyone searches for more information about the tragic shooting, we start our morning look at American politics with President Donald Trump’s weekend, when he thumbed out nearly 30 tweets — most of them from his country club in New Jersey.

Seventeen of Trump's tweets defended his administration’s handling of the hurricane aftermath in Puerto Rico and the Caribbean. “The Fake News Networks are working overtime in Puerto Rico doing their best to take the spirit away from our soldiers and first R's. Shame!”

Four attacked the mayor of San Juan, Puerto Rico, who has criticized the administration’s response. “Such poor leadership ability by the Mayor of San Juan, and others in Puerto Rico, who are not able to get their workers to help,” Trump said. “They want everything to be done for them when it should be a community effort. 10,000 Federal workers now on Island doing a fantastic job.”

Three more tweets focused on North Korea, which appeared to undermine his State Department opening communications to that regime. “I told Rex Tillerson, our wonderful Secretary of State, that he is wasting his time trying to negotiate with Little Rocket Man,” Trump tweeted. “Save your energy Rex, we'll do what has to be done!”

Two focused on the national anthem and the controversy over NFL players taking a knee. “Very important that NFL players STAND tomorrow, and always, for the playing of our National Anthem. Respect our Flag and our Country!”

Related: Trump Tweet Undermines Tillerson on North Korea Talks

And one recapped last week’s GOP Senate runoff in Alabama. “In analyzing the Alabama Primary race, FAKE NEWS always fails to mention that the candidate I endorsed went up MANY points after endorsement!” (In fact, Trump’s candidate Luther Strange, trailed by six points in the initial vote after Trump’s endorsement, 39 percent to 33 percent, and he ultimately lost by 10 points on Tuesday, 55 percent to 45 percent.)

Let our news meet your inbox. The news and stories that matters, delivered weekday mornings. This site is protected by recaptcha

Here’s the political reality of those tweets: They aren’t helping his presidency. At all. According to last month’s NBC/WSJ poll, 66 percent of Americans disapprove of Trump’s use of Twitter, and that includes 87 percent of Democrats, 71 percent of women, 64 percent of independents, 61 percent of men, 59 percent of seniors, 46 percent of Republicans and even 36 percent of self-described Trump voters.

FiveThirtyEight: Trump’s approval rating goes down when he rage-tweets

Indeed, FiveThirtyEight has analyzed that Trump’s approval rating typically ticks up after he stays away from Twitter, and it goes down when he rage-tweets.

“If Trump never tweeted again, his approval rating would gradually rise to about 43 percent, the simulation estimates. Conversely, if he went on a Twitter bender and constantly tweeted at his maximum outrage level … it would eventually fall to 33 percent. Again, I wouldn’t take any of this too seriously. But Trump’s tweets often dictate news cycles and amplify controversies — and they can even help to spark diplomatic crises and put the president in legal jeopardy. It’s not crazy to think the tweets have had consequences — mostly negative ones — for Trump’s popularity.”

An NBC analysis looking at Trump’s first 100 days made the same conclusion: There’s a correlation between his tweetstorms and his negative ratings.

The latest on the mass shooting in Las Vegas

NBC News: “A gunman fired a barrage of bullets on an outdoor country music festival in Las Vegas Sunday night, killing at least 50 people and injuring more than 400 others, police said, in the worst mass shooting in modern American history.

“From his room on the 32nd floor of a glitzy hotel, the shooter, identified by law enforcement officials as Stephen Paddock, 64, of Mesquite, Nevada, fired shot after shot down on the crowd of more than 22,000, sending terrified concertgoers running for their lives.”

President Trump issued this statement via Twitter: “My warmest condolences and sympathies to the victims and families of the terrible Las Vegas shooting. God bless you!”

U.S. Supreme Court's 'momentous' term begins today

NBC’s Pete Williams previews the U.S. Supreme Court’s “blockbuster docket” for its upcoming term, which begins today.

“Objections to same-sex marriage, state regulation of sports betting, the privacy of cellphone users and limits on political partisanship dominate the court's agenda.”

“‘There's only one prediction that's entirely safe about the upcoming term,’ Justice Ruth Bader Ginsburg said at Georgetown University's law school late last month. ‘And that is it will be momentous.’”

Mnuchin doesn't rule out tax increases for some Americans under GOP tax plan

On "Meet the Press" yesterday, Treasury Secretary Steve Mnuchin wouldn't rule out that the GOP tax plan would RAISE taxes on some Americans.

"[A]s you know, one of the things we're trying to do is eliminate lots and lots of deductions. One of those deductions is about getting the federal government out of the business of subsidizing the states. So this will have different impacts to people in different states. And we're sensitive to that."

Indeed, the nonpartisan Tax Policy Center said about 12 percent of taxpayers (especially those making between $150,000 and $300,000 because itemized deductions would be repealed) would see an average tax increase of $1,800 under the GOP’s tax plan.

Meanwhile, retiring Sen. Bob Corker, R-Tenn., said he would oppose the proposal if it adds “one penny to the deficit.”

“I'm willing to accept a reasonable score on dynamic growth. A reasonable score on dynamic growth. Joint tax does those. There're some other groups that do it. But if it looks like to me, Chuck, we're adding one penny to the deficit, I am not going to be for it, okay? I'm sorry. It is the greatest threat to our nation. The greatest threat to our nation,” Corker said on “Meet the Press.”