Here are highlights from CNN's Republican presidential debate Dec. 15 in less than two minutes. (Sarah Parnass/The Washington Post)

The final Republican debate of the year featured sharp exchanges over national security, personal insults and regular interruptions, but in the end there were no outright winners. In that sense, it was an almost perfect reflection of the party’s unpredictable nomination campaign.

The two-hour session was as unruly as it was substantive. It dealt with some of the most serious issues of the moment but broke down in shouting and interruptions that seemed to underscore the determination of all the candidates to make their mark as the campaign heads into the holidays and a short respite before resuming in January.

Donald Trump, who dominates the national polls, came under repeated fire for proposing to bar the entry of Muslims into the United States and for saying he would go after the families of Islamic State terrorists. Though appearing flustered at times, he held firm on his positions and refused to concede any ground, seemingly confident that his harsh rhetoric continues to find support.

Former Florida governor Jeb Bush, whose support nationally has eroded steadily through the fall, led the attacks against Trump, a sign of his resolve to make one more run at turning around a candidacy that has struggled for visibility. More than any other candidate, he got under Trump’s skin — but without a clear outcome.

Last night’s debate was only the penultimate lap for the GOP candidates, not the finish. The Washington Post’s Robert Costa talks about about which candidates were the strongest and which still have a long road ahead. (Alice Li/The Washington Post)

Sens. Ted Cruz (Tex.) and Marco Rubio (Fla.), who have been circling each other for weeks in anticipation of a final showdown sometime next year, argued over security, surveillance and the best strategy to defeat Islamic State terrorists. Cruz was thrown on the defensive over security issues, and both had to defend their positions on immigration. But neither candidate scored a truly telling blow on the other.

The debate was singularly focused on matters of war and terrorism, coming at a time of heightened security concerns and on a day when schools in Los Angeles were closed because of a threat of a terrorist attack.

The discussion enjoyed only one clear consensus: that President Obama and Hillary Clinton have left the country less safe. The conversation also revealed significant divisions within the Republican Party on issues that GOP voters now see as the most important for the coming election.

Everyone onstage seemed eager to make a mark.

New Jersey Gov. Chris Christie, rising in New Hampshire, sought to belittle Cruz and Rubio, but he mostly attacked Obama and Clinton, while brandishing his credentials as a former prosecutor in the fight against terrorism.

Sen. Rand Paul (Ky.) staked out his position as the least hawkish of the group, while effectively inserting himself into the back and forth between Cruz and Rubio. Carly Fiorina sought to highlight her private-sector experience. Ohio Gov. John Kasich called for a candidate who could unify the country. And retired neurosurgeon Ben Carson sought to counter impressions that his inexperience in foreign affairs makes him a poor candidate for the presidency.

At some moments, the debate delved into arcane, though important, details of how to combat terrorist threats — touching on Islamic State fighters abroad, the possibility of homegrown terrorists becoming radicalized without family or neighbors knowing, the proper vetting of Syrian refugees coming to the United States, and how best to secure the country’s borders.

At other points, the frustrations of a campaign that has defied predictions and expectations, one in which governing experience has proved to be no asset and outsiders have drawn significantly more support, spilled onto the stage.

The exchanges between Bush and Trump were particularly revealing as a measure of how the race has unfolded in ways the former governor never expected. Bush sought to cast himself as the grown-up and Trump as an unserious candidate. He called Trump’s proposals “crazy” and “unhinged.”

Trump, as has been his style all year, swatted back by talking about his strength vs. what he said was Bush’s weakness. In trying to dismiss Bush, he compared his front-running status in the polls with Bush’s single-digit support.

The back and forth between Cruz and Rubio was far more substantive but with a political edge. Rubio’s goal was to undermine Cruz’s effort to consolidate conservatives of all stripes — tea party, evangelical and libertarian — by questioning whether he is as conservative as he claims. Cruz’s rebuttals were designed to paint Rubio as an establishment conservative, out of touch with an electorate that includes many voters angry at the party’s leadership.

The exchange everyone was expecting — between Cruz and Trump — came and went with no points scored. Cruz had criticized Trump at a private fundraiser last week, suggesting that there were legitimate questions about whether the billionaire businessman had the judgment to be president.

When the issue was raised near the end of the debate, a smiling Trump mockingly warned Cruz not to attack. Cruz ducked the question, saying the voters will have to make judgments about all of the candidates’ qualifications, temperament and judgment. Once again, the two men — who have avoided going after each other — maintained their unusual alliance.

The debate marked the end of one of the most tumultuous and unpredictable years in American politics. Whether it was a harbinger of things to come or a false indicator of the state of the Republican Party is the question that will begin to be answered in seven weeks.

The lineup on the stage highlighted how, in 2015, grass-roots Republicans have rejected the governing class in favor of outsiders. Trump and Carson, two nonpoliticians, and Cruz, an anti-establishment senator, stood together as a symbol of the electorate’s fury with the established order.

Arranged farther out on the stage were a series of elected officials — among them two governors, a former governor and two senators — who have struggled to find their balance in a year when records in office, governing experience and policy white papers have been given short shrift by rank-and-file conservatives.

That’s been the theme of 2015: the establishment on the defensive and outsiders on the rise. But what comes next is the question all the candidates and many in the party leadership are asking.

Months ago, many Republican strategists assumed that the nomination contest would begin to revert to more familiar form as the primaries and caucuses neared. But that was before Trump survived one controversy after another, the latest coming in the past week with his proposal to temporarily bar Muslims from entering the United States. If the recent polls are correct, Trump has weathered that storm as he has survived others.

Now the uncertainty about what comes next seems greater than ever. Could Trump be the nominee? Will Republicans see a three- or four-person race into the spring, with Trump in the center of the action? Or will the campaign devolve to a two-person contest between Cruz and Rubio, as some have predicted recently? Will New Hampshire send one of the other establishment candidates into the later contests with enough momentum to become a force in the race?

Along with uncertainty comes a sense of urgency for all the candidates. When the campaign resumes in earnest after the holidays, Iowa’s caucuses will be only a month away and New Hampshire’s primary a short five weeks in the future.

Many of the candidates hope or assume that the polls will look different by late January and that Iowa or New Hampshire will deliver a surprise. History suggests that could be the case. But history has been an unreliable guide in this pre-election year. That’s why Tuesday’s debate looked and sounded the way it did.