By Sahil Kapur, Bloomberg News (TNS)

WASHINGTON — Just four weeks before the first votes of the 2016 presidential contest are cast in Iowa’s caucuses, a bizarre, unpredictable year in American politics comes to an end on Thursday night. With billionaire reality TV star Donald Trump and tea party-aligned Sen. Ted Cruz leading the Republican field, the big question is whether the 2016 election will change American politics as we know it or whether there will be a return to the familiar in the final stretch.

Here are six factors that could make a difference.

1. Cruz loves the campaign trail

Cruz is as comfortable and happy on the stump as he is uptight and gloomy on Capitol Hill. While Republican rivals like Florida Sen. Marco Rubio, former Florida Gov. Jeb Bush and Ohio Gov. John Kasich approach campaigning like a job, Cruz revels in it, often coming across as a boisterous televangelist feeding off the adoration of the crowd. The Texas senator deploys fiery lines and sarcastic jokes from one campaign stop to the next with an identical script and delivery, right down to the pauses for applause and impact, like an actor who has practiced and perfected a routine in the mirror the night before.

2. Rubio’s eyes are on the November prize

Rubio’s pitch to Republican voters is focused on his electability. Campaign allies who introduce him on the stump make a point to mention it, and the Floridian likes to bring it up himself. Rubio’s message of a “new American century” and picking a leader for “tomorrow” and not “yesterday” sneak-previews themes he hopes to use against Democratic front-runner Hillary Clinton in a general election.

That’s why, even as he sounds the conservative note meant for the voters who tend show up at Republican caucuses and primaries, Rubio maintains ideological escape hatches on a number of hot-button issues. For instance, on immigration reform, he emphasizes border security hurdles he’d erect as a prerequisite but he’s open to a path to citizenship for those now living here illegally. On abortion, Rubio says he’d prefer to enact an across-the-board ban, but says he’s willing to support popular exceptions for rape and incest. He doesn’t want to take in new Syrian refugees, but softens that line when it comes to young orphans or elderly widows.

This tendency toward triangulation has bred distrust in some corners of the right where Rubio is seen as too cozy with party elites. But another way to look at it is Rubio is keeping one eye trained on the fall campaign, when winning means attracting voters in the center. Amassing Republican cred does Rubio little good: He says he’s quitting the Senate no matter what happens in the election (his term expires after 2016), so he’s betting his political career on November.

3. The big difference between Cruz and Rubio: temperament

Much has been made of the differences between the Senate’s two Cuban-American freshmen on immigration and national security. Cruz has ruled out legalizing people in the U.S. illegally, while Rubio remains open to a path to eventual citizenship for undocumented immigrants. Rubio espouses a foreign policy of promoting democracy abroad, while Cruz opposes nation-building. Rubio has attacked Cruz for supporting legislation that stopped the government from automatically collecting bulk phone data, although Cruz argues that it protects Americans from terrorist attacks while also protecting civil liberties.

Apart from those disputes, however, there are precious few policy disagreements that separate the two top-tier candidates. The two first-term senators have similar voting records and are rated by right-of-center groups as among the most conservative senators in recent history.

Temperamentally, however, they’re opposites, and that contrast shows on the campaign trail. While Cruz deploys scorched-earth rhetoric calling for a conservative revolution, Rubio offers a gentler touch in calling for a course correction to preserve the American dream. Cruz is appealing to the hard- core conservative wing that has grown in influence during the Obama era. Rubio is trying to court the tea party base that fueled his 2010 Senate bid while also appealing to old-school moderates in the Republican tent, who may not be as loud or excitable but tend to vote regularly and have powered past nominees such as Mitt Romney in 2012 and John McCain in 2008.

4. The rise of the Islamic State is helping Chris Christie

Not coincidentally, the New Jersey governor’s support among Republicans in New Hampshire and nationally has grown since the San Bernardino, Calif., shootings elevated fears of terrorist sleeper cells. A former federal prosecutor who has been accused of embellishing his credentials, Christie has a campaign message that hearkens back to George W. Bush’s 2004 campaign. “This country is in danger and we need a president who’ll protect the American people,” Christie said recently at a stop in Exeter, New Hampshire. “The world is coming apart at the seams, and we can feel it.”

It’s hard to find anybody at Christie rallies in New Hampshire, the early state where he’s placed most of his focus, who doesn’t cite national security as a top concern. The higher that number goes with Republican voters, the more room Christie has to sell his message and grow his base. And he does so with a unique touch — a New Jersey bravado that tells voters he’s a tough guy but also has an empathy that says he cares about them and understands their fears. That could be an effective combination.

5. Trump needs his angry, alienated base to show up

It’s not hard to find people at Trump rallies who think President Barack Obama is a Muslim (he is a Christian). Placards have compared the commander in chief to Neville Chamberlain, the prime minister of Great Britain who infamously tried to appease Adolf Hitler. Some spout darker conspiracy theories that the president wants to help radical Muslims destroy the U.S. Underlying the profound disaffection among Trump’s predominantly white base of supporters: concerns about an economy that has left blue-collar workers behind and about demographic changes that are projected to eventually make whites a minority in the nation.

That may account for Trump’s staying power, defying endless predictions of impending doom, and looming as the single biggest and most consequential surprise of 2015. The million-dollar question in 2016 is whether the estranged and alienated voters Trump has attracted will show up at the polls that count. The answer to that question will determine whether Trump has changed American politics as we know it.

6. Bush is becoming an anti-Trump message candidate

Some candidates may never win, but want to be on the presidential stage to spread a message they care about, whether it’s Ron Paul touting an idiosyncratic brand of isolationist libertarianism or Jesse Jackson crusading for civil rights and workers’ rights. One of the more unexpected twists of 2015 has been the evolution of Bush — a son and brother of former presidents who entered the race as a favorite for the nomination — into something of a message candidate as his poll numbers collapse. His message: Trump is dangerous and “unhinged,” would be a “chaos president,” and would lose to Clinton anyway.

©2015 Bloomberg News. Distributed by Tribune Content Agency, LLC.

Photo: Ted Cruz, Jeb Bush and Donald Trump talk at the end of the debate. REUTERS/Mike Blake