By Stephen Collinson

CNN

WASHINGTON (CNN) -- President Donald Trump will spend his next 100 days trying to get things done that he once confidently predicted he would accomplish in his first 100 days in office.

The White House spent last week fending off unflattering critiques of Trump's first three months in the Oval Office, claiming he had racked up great achievements while also dismissing what it sees as an artificial milestone.

But the experience of the first 100 days has shown the enormity of the challenge Trump faces in enacting his proposed policies amid partisan acrimony in Washington, where Democratic opposition is determined to thwart him and Republican infighting persists.

The looming political fights of the next three months will test whether Trump has learned from his mistakes in the first 100 days and can find a way to exert his will over Congress.

Abroad, the President faces several deepening crises, most immediately the showdown with North Korea over its nuclear and missile programs.

The next 100 days will also see him venture overseas for the first time as President, and may begin to reveal exactly what he means when he talks about an "America First" foreign policy. Meanwhile, the world is watching to see whether Trump pulls America out of the Paris climate accord -- a decision that is expected within the next several weeks.

Trump geared up for the next chapter of his presidency on Saturday night, surrounding himself with adoring supporters in Pennsylvania, and conjuring up the spirit and fury of his 2016 election campaign, by bashing Washington elites and the media.

"Their priorities are not my priorities and not your priorities," Trump told the crowd in Harrisburg. "If the media's job is to be honest and tell the truth, the media deserves a very, very big, fat, failing grade."

The remarks were a clear sign that the President intends to leverage the support of his loyal political base against the Washington establishment in an effort to kickstart his stalled agenda. There does not seem to be any imminent plan being developed to reach out to his critics and broaden his appeal.

The President's most immediate problem is over Obamacare: He must find a way to pilot the repeal bill, which remains in limbo, neither alive, nor dead, through the House of Representatives. The failure to realize his number one campaign promise during the first 100 days dealt a humiliating blow to Trump and raised questions about his presidential authority.

The White House has repeatedly predicted in recent days that a vote on the legislation is imminent, but continued wrangling between conservatives and Republican moderates on the bill is delaying its passage.

But the House vote will only be the first hurdle for the bill, with the Senate expected to fundamentally change the legislation which Trump says must cut premiums and broaden access, but which a Congressional Budget Office report found would deprive millions of people of coverage. And even if the White House can record a political win by passing the bill, Trump will likely be saddled with the blame if Americans come to believe that the new disruption in the health care industry costs them money or access to insurance.

Fighting the Republican Congress

The health care fight has revealed an unexpected characteristic of Trump's Washington. Primary opposition to the President has not come from Democrats, who were expected to form a road block to the White House, but from his own party. And unless Trump can find a way to mobilize the Republican monopoly on power on Capitol Hill, the next 100 days could be as barren as his first 100 in terms of significant legislation.

"I think the rules in Congress, and in particular the rules in the Senate, are unbelievably archaic and slow-moving and, in many cases, unfair," Trump said Sunday on CBS's "Face the Nation."

"In many cases, you're forced to make deals that are not the deal you'd make. You'd make a much different kind of a deal. You're forced into situations that you hate to be forced into."

He made sure, however, to praise Hill Republicans, telling CBS they are "great people" and suggested they were learning how to govern now that they fully control the levers of power in Washington.

Still, House Speaker Paul Ryan is insisting that despite a turbulent and unproductive start, Republicans will come together to pass Trump's agenda.

"I talked about 200 days because I thought the kind of agenda that we're attempting to put together here -- overhauling health care, overhauling the tax system, rebuilding our military, securing the border -- those take more than just a few months," Ryan told reporters last week.

"They take a long time, at least a year."

Tax reform won't be easy

While the White House has struggled to sell and pass the health care bill, the tax reform effort could be even more complicated. The administration rolled out an outline of a plan last week, which includes large tax cuts for corporations and the wealthiest Americans, and will use the next few months to sell it to the American people.

Winning public support for the bill will be one thing. Getting it passed will be even more complex because Trump will have to woo some Democrats to navigate the measure through the Senate, and there is no sign Minority Leader Chuck Schumer's troops are ready to help the President get a win.

If Democrats refuse to cooperate, the White House could seek to pass the bill with a simple majority in a maneuver known as reconciliation. That route brings an added complication, because any legislation passed in this manner must not add to the deficit. Vice President Mike Pence admitted on Sunday that "maybe in the short term" the tax reform bill will increase the deficit but argued that there was no alternative to stimulating economic growth.

"The truth is, if we don't get this economy growing at 3% or more, as the President believes that we can, we're never going to meet the obligations that we've made today," Pence said on NBC's "Meet the Press."

The White House's ability to manage Congress will also be tested as work starts on Trump's 2018 budget, which includes large cuts to the State Department, the Environmental Protection Agency and foreign aid -- some of which are likely to stir opposition even inside Trump's own party.

The President also faces a challenge in the coming months in securing funding for his border wall, amid opposition from some Republicans in states affected by the project. Some conservatives may also balk at spending billions of dollars to pay for a campaign promise that Trump vowed would be financed by Mexico.

The President, however, is refusing to back down on the need for a wall, that formed a symbolic foundation for his presidential campaign and is highly popular with his political base.

"We'll build the wall, folks," Trump said in Harrisburg. "Don't even worry about it. Go to sleep. Go home, go to sleep, rest assured."

North Korea showdown, visit to Europe

Trump's challenges abroad over the next 100 days are dominated by the sharpening confrontation with North Korea. The period will likely reveal whether his strategy for containing the threat from Pyongyang --- based on pressuring China to rein in its ally --- will work in the medium term.

Trump has warmly praised China's President Xi Jinping for upping pressure on North Korean leader Kim Jong Un. But in the past, Beijing has shown that there are limits to the steps it will take to constrain Pyongyang. Another nuclear test by North Korea, which could come at any time, would likely further escalate the crisis.

The President will not travel to Asia until the fall, on a trip likely to be consumed by the North Korea showdown. But first, and within the next 100 days, he will travel to Europe, which is still trying to size up the new US President following derogatory comments about the European Union and NATO during his campaign that rattled America's allies.

The President is due to attend the NATO summit in Brussels next month.

To the relief of key European powers, Trump has stepped back from his anti-NATO rhetoric, but is still certain to demand that alliance members do more to share the financial burdens of their own defense.

Trump will also travel to Italy for the G7 summit, where his protectionist rhetoric will mark a sea change for the rich nations club which has traditionally backed free trade. Trump's moves to roll back environmental regulations introduced by the Obama administration to tackle global warming have also alarmed European governments, one reason why his decision on the Paris agreement is so keenly awaited.