WASHINGTON — President Donald Trump's unorthodox quest for the White House was fueled by his disregard for convention and the nagging sense that he — and ultimately his supporters — were underestimated and disrespected.

Now the 45th president, Trump and his team are determined to deliver quickly on promises — on the economy, health care, tax reform and immigration — critics told him he could not possibly fulfill.

During his campaign, Trump embraced the notion of a first 100-day flurry in which he would quickly put his stamp on Washington. In a major speech in Gettysburg, Pa., a few weeks before the election, he articulated three broad priorities: ending “corruption and special interest collusion” in Washington, protecting American workers and restoring security and “constitutional rule of law.”

On his first day in office, he said he would take more than a dozen specific actions to advance priorities: introducing a constitutional amendment to impose congressional term limits, starting to renegotiate the North American Free Trade Agreement, and canceling federal money going to so-called sanctuary cities among others.

Other legislative priorities would require congressional support: simplification of the tax code, a major infrastructure bill and tax credits for child and elderly care.

“If we follow these steps, we will once more have a government of, by, and for the people,” he said.

Separately, the new administration has been coordinating with Republican leaders on how to take advantage of the congressional calendar to fill a vacancy on the Supreme Court, start the repeal of the Affordable Care Act and later push reform of taxes and Medicare and Social Security and ramp up immigration enforcement through the appropriations process ahead of a late-April deadline.

“That's a first 100 days that I'm not sure any other conservative president has hit those kind of major milestones,” said James Wallner, vice president for research for the Heritage Foundation, which has consulted closely with the Trump team.

Trump takes office in a far different environment than President Barack Obama encountered eight years ago. Then, an economy still in free-fall required an urgent response on multiple fronts – preparing an economic stimulus package, debating a rescue of the auto industry, devising a response to the housing crisis, among other domestic priorities.

Trump inherits a relatively stable and growing economy and no crises demanding his immediate attention, giving him and his team more freedom to roll out an agenda on their own terms.

But many conservatives view the task of reversing Obama's agenda as urgent on its own. Republican lawmakers are primed to send scores of bills to Trump's desk as he also considers executive actions to speed a rightward turn in government.

But like many of Trump's predecessors discovered, the playbook that delivered him to the White House is not easily adapted to an office that has confounded even the most prepared and popular occupants.

“It's not learning how to hit a 98-mile-an-hour fastball coming from a Triple-A ball club. It's learning how to hit a 1,200-mile-an-hour fastball,” said Terry Sullivan, executive director of the White House Transition Project, who has closely studied presidents' first 100 days.

Still, presidencies that have stumbled most in the early days owed their misfortune not to any deficit in public support, but to their own decisions, Sullivan said. Most often, problems arise when a new president succumbs to temptation to react to events, squandering the most precious resource a president has: time.

“There are an enormous number of things clamoring for the president's attention, and none of them are unimportant,” he said. “If he's not moving his agenda forward, it's not standing still, it's drifting away.”

michael.memoli@latimes.com