New Hampshire was just about the ideal state for Nikki Haley. It doesn’t get better from here.

Is the Republican presidential primary over already?

Not quite, but it’s a reasonable question after New Hampshire’s first-in-the-nation primary delivered a clear victory for Donald J. Trump on Tuesday night. And if your definition of “over” is whether Mr. Trump is now on track to win without a serious contest, the answer is probably “yes.”

With nearly all the counting done, he won 55 percent of the vote. His only remaining rival, Nikki Haley, won 44 percent.

Mr. Trump’s 11-point margin of victory is not extraordinarily impressive in its own right. In fact, he won by a smaller margin than many pre-election polls suggested.

What makes Mr. Trump’s victory so important — and what raises the question about whether the race is over — is that New Hampshire was Ms. Haley’s very best opportunity to change the trajectory of the race. It was arguably her very best opportunity to win a state, period.

If she couldn’t win here, she might not be able to win anywhere — not even in her home state of South Carolina, where the race turns next. And even if she did win her home state, she would still face a daunting path forward.

Mr. Trump leads the national polls by more than 50 percentage points with just six weeks to go until Super Tuesday, when nearly half of all the delegates to the Republican convention will be awarded. Without an enormous shift, he would secure the nomination in mid-March.

Why was New Hampshire such an excellent opportunity for her?

• The polls. New Hampshire was the only state where the polls showed her within striking distance. She trailed by a mere 15 points in the state, compared with her 50-plus-point deficit nationwide. She isn’t within 30 points in any other state, including her home state of South Carolina.

• History. The state has a long track record of backing moderate and mainstream Republican candidates, including John McCain and Mitt Romney. Mr. Trump won the state with 35 percent of the vote in 2016, but mostly because the moderate vote was divided.

• The electorate. Ms. Haley fares best among college graduates and moderates, and the New Hampshire electorate is full of those voters. The state ranks eighth in the college-educated share of the population, and unlike in many states, unaffiliated voters are allowed to participate in the Republican primary.

• The endorsements. In contrast with most states, New Hampshire’s political elite did not coalesce behind Mr. Trump. Ms. Haley even had the support of the state’s popular Republican governor, Chris Sununu.

• The media. New Hampshire’s first-in-the-nation primary receives far more media attention than later contests. It offered the possibility — if only a faint one — that a win could change her fortunes elsewhere. A later victory in a similar state like Vermont — whose Republicans also tend to be more moderate — could be drowned out by other primary results that day and dismissed as too-little-too-late.

Ms. Haley made good on all of these advantages Tuesday. She won 74 percent of moderates, according to the exit polls, along with 58 percent of college graduates and 66 percent of voters who weren’t registered Republicans.

But it wasn’t close to enough. Ms. Haley lost Republicans by a staggering 74 percent to 25 percent — a group of no small import in a Republican primary, especially in the states where only registered Republicans can vote. Conservatives gave Mr. Trump a full 70 percent of the vote. Voters without a college degree backed Mr. Trump by 2 to 1.

In other Republican primaries, numbers like these will yield a rout. Conservatives, Republicans and voters without a degree will represent a far greater share of the electorate. There is no credible path for her to win the nomination of a conservative, working-class party while falling this short among conservative, working-class voters.

Worse, Ms. Haley’s strength among independents and Democrats will make it even harder for her to expand her appeal, as Mr. Trump and other Republicans will depict her campaign as a liberal Trojan horse.

If Ms. Haley had won New Hampshire, the possibility of riding the momentum into later states and broadening her appeal would have remained. Not anymore. Instead, it’s Mr. Trump who has the momentum. He has gained nationwide in polls taken since the Iowa caucuses. Even skeptical Republican officials who were seen as Ms. Haley’s likeliest allies, like Tim Scott or Marco Rubio, have gotten behind the former president in recent days.

Whether the race is “over” or not, the New Hampshire result puts Mr. Trump on a comfortable path to the nomination. The Republican Party’s rules for awarding delegates, which allow states to award all of their delegates to the winner, could let him clinch the nomination in early March. Mr. Trump’s legal challenges add an extra twist — if he’s convicted of a crime, perhaps he’ll lose the nomination at the convention. But by the usual rules of primary elections, there’s just not much time for the race to change. If it doesn’t, Mr. Trump could easily sweep all 50 states.