President Trump ushered in 2019 in characteristic fashion -- with a tweet expressing supreme confidence in himself, contempt for the “fake news media” and optimism for the country.

The famously teetotaling commander-in-chief proved he wasn't nursing a New Year's hangover with the early morning missive, his second of the freshly-minted year. The first was a message praising former adviser Sebastian Gorka, who has a book out.

“Happy new year to everyone, including the haters and the fake news media!” the president tweeted. “2019 will be a fantastic year for those not suffering from Trump derangement syndrome.”

Trump added, “Just calm down and enjoy the ride, great things are happening for our country!”

Trump also used Twitter to fire back at a critic who had slammed him on television days earlier, labeling retired Army Gen. Stanley McChrystal a "Hillary lover!" with a “big, dumb mouth." McChrystal, appearing on ABC's "This Week," said Trump is immoral and doesn't tell the truth.

The first tweets of the year indicate the president will not slow down his social media activity in the new year. Trump enters 2019 facing a number of challenges: a partial government shutdown, a volatile stock market, Democrats taking control of the House, a forthcoming report from Special Counsel Robert Mueller’s investigation, White House staff turnover and the kick-off to his 2020 re-election campaign.

The partial federal government shutdown enters its second week – and the new year – without a deal to re-open the government in sight, as both sides continue digging in over funding for a barrier on the border with Mexico.

House Democrats plan to introduce a legislative package to re-open the government once they get control of the House on Thursday, but it's not clear what kind of support it will get from Republicans. Trump has said he will only support a bill with funding for his border wall.

"The Democrats, much as I suspected, have allocated no money for a new Wall," the president tweeted Tuesday. "So imaginative! The problem is, without a Wall there can be no real Border Security - and our Country must finally have a Strong and Secure Southern Border!"

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On Wall Street, while other signs indicate the economy is strong, U.S. stocks wrapped up a volatile year and month as a number of worries weighed on investor sentiment.

The 30-stock Dow Jones Industrial Average and the broader S&P 500 both ended the year about 7 percent lower, while the tech-heavy Nasdaq Composite closed out 2018 about 4 percent down – its biggest one-year decline since 2008.

On Capitol Hill, the incoming Democratic majority in the House of Representatives has the power to open a slew of investigations into the White House and President Trump when the new Congress is seated this week, and early indications are that Democrats plan to aggressively take advantage of their new authority.

The president is also preparing for the completion of a report by Mueller, whose team has been investigating whether anyone from Trump’s campaign colluded with the Russians during the 2016 election, and other issues.

Mueller's findings might provide a launching point not only for further investigations but for even impeachment proceedings from House Democrats. While some in the party have already pushed for impeachment, Democratic leaders have yet to embrace the effort.

The president also faces the challenge of staffing his administration and getting his new Cabinet picks confirmed from the Senate, amid departures of the White House chief of staff, the secretary of defense, his attorney general, and other positions.

TRUMP: WARREN'S 'PSYCHIATRIST' KNOWS WHETHER SHE THINKS SHE CAN WIN WHITE HOUSE IN 2020

Secretary of Defense Jim Mattis, who submitted his resignation on Dec. 20 and was, in effect, fired by Trump three days later, is being replaced by Deputy Secretary Patrick Shanahan. Shanahan, a former Boeing executive, will be acting defense secretary until someone is nominated for the post.

Trump is also gearing up for re-election, as several dozen Democrats could launch campaigns to challenge him in 2020. On Monday, Massachusetts Sen. Elizabeth Warren announced the formation of her exploratory committee.

The president wasted no time in attacking her, reviving claims Warren repeatedly lied about her heritage to obtain affirmative-action benefits in the course of her academic career.

In an interview with Fox News' Pete Hegseth, Trump was asked whether Warren really thinks she could make him a one-term president. "Well, that I don't know," Trump responded. "You’d have to ask her psychiatrist."

Fox News’ Greg Re and Fox Business’ Mike Obel and The Associated Press contributed to this report.