Seven Republican presidential candidates will meet on stage in Charleston, S.C. on Thursday. Here's what you need to know before the debate. (Peter Stevenson/The Washington Post)

Seven candidates. Eighteen days before the Iowa caucuses. Donald Trump's continued dominance. The stakes are high, and the political drama is expected to be as well in Republicans' sixth main presidential debate, held in South Carolina on Thursday night.

Here's what you need to know about the six big issues expected to be in play:

1. Terrorism and the Islamic State

With back-to-back high-profile attacks this week in Istanbul and Jakarta that appear linked to the Islamic State, terrorism and national security remain high on the 2016 topics list.

[Get updates on tonight's debate with our live blog]

Polls show that a large majority of Americans are fearful of another terrorist attack and doubtful of President Obama's ability to handle it, prompting Obama to go on the offensive to highlight how much the United States is, well, on the offensive in Iraq and Syria to battle the Islamic State.

The national focus on terrorism ostensibly works in the Republican Party's favor. GOP candidates traditionally have earned more trust on national security issues from voters than Democrats have. But the most hawkish among the bunch, Sen. Lindsey O. Graham (S.C.), dropped out of the race after the last debate. He wanted to send as many as 10,000 troops on the ground. No other candidates has gone that far, especially as Republicans recall the war-weariness that marked the drawdowns in Iraq and Afghanistan.

So although their tone will be different than Obama's -- including on whether to call it "radical Islam" -- their actual proposals have in many cases not differed in big and clear ways.

2. U.S.-Iran relations

News that 10 U.S. sailors drifted into Iranian waters and were held by the Iranian government for several hours Tuesday has brought the contentious Iran nuclear deal -- and broader relations between the pariah country and the United States -- back to the forefront in Washington.

Republican candidates criticized Obama for not mentioning the sailors in his State of the Union address on Tuesday. Less than 24 hours after Iran released the sailors early Wednesday, House Republicans approved a bill that would undermine the historic nuclear agreement the United States and other countries reached with Iran this summer. The bill would require certain Iranians and financial institutions to prove they're not involved in Iran's ballistic missile program or terrorist activities. It's aimed at restricting the sanctions the United States will begin lifting on Iran as a result of a nuclear agreement.

Obama is certain to veto the bill if it ever comes to his desk -- which hit a procedural snafu and will have to be voted on again later in January anyway. But it highlights the dearth of trust between the two nations, and how Republicans almost universally agree that our current path with Iran is a bad one.

3.  Ted Cruz's bank loan (and birthplace)

The latest candidate to have to answer for a past discrepancy is Cruz -- one of the leaders in Iowa. The New York Times looked at his personal financial disclosures from 2012, when he was running for the Senate bid that launched his national career, and reported Thursday that he received about $1 million in loans from Goldman Sachs, his wife's former employer, to finance his race, but didn't disclose the loan in federal campaign finance reports as required.

Cruz isn't the first candidate and won't be the last to misreport campaign finances. What's more concerning for him is the optics this story presents; he has worked hard to pitch himself as a populist, but now he has to defend ties to Wall Street.

It's just the opening Trump needs to chop at Cruz's supposed outsider status and continue the Trump-Cruz feud.

Also on the radar for Cruz is Trump's repeated questioning of whether Cruz is eligible to serve as president, given he was born in Canada. Cruz was born to an American mother, so he is a U.S. citizen, but not all legal scholars agree that makes him a "natural-born citizen" -- the standard for serving as president. And the issue is untested in courts.

This will be Cruz's chance to try and put the matter to rest in front of voters who may be confused or uncertain.

4. The economic recovery

In his final State of the Union address, Obama stressed how, under his watch, the United States was pulled back from the brink of a depression and is stronger for it.

"The United States of America, right now, has the strongest, most durable economy in the world," he said, ticking off record-setting private-sector job creation, the national unemployment rate being cut in half and the deficit being trimmed by almost three-quarters.

Republican candidates aren't so quick to call that a win, especially when voters express skepticism that the economy is improving. To counter Obama's numbers, GOP candidates often point to the number of people who have decided not to look for work anymore. The labor force participation rate keeps dropping and now stands at 62.6 percent -- down from more than 66 percent before the recession.

And a nonpartisan National Association of Counties analysis released this week reports that 93 percent of American counties have yet to fully recover from the recession, with about half recovering on one or fewer of four key indicators.

The darker the blue, the better a county's economic recovery. (Courtesy National Association of Counties).

On how they'd reboot the economy, GOP candidates have drawn more contrasts with Obama than with one another. Specifics are sparse, and the claims from governors -- such as Ohio Gov. John Kasich and former Florida governor Jeb Bush -- of job growth in their states have come under scrutiny by fact checkers. The Washington Post's Fact Checker team urges readers "to be wary about job-creation claims, either at the state or national level, as so much of what happens in an economy is beyond a politician’s control."

That probably won't stop those claims from being bandied about on Thursday, though.

5. Immigration

The perennial topic in the GOP primary is a particularly troubling topic for the candidate in third place behind Trump and Cruz, Sen. Marco Rubio (Fla.). In part because of his standing in the polls, multiple Rubio rivals are going after his record in the Senate in increasingly personal attacks.

In particular, they're zeroing in on Rubio's role in helping draft an immigration reform bill that passed the Senate in 2013 and included a path to citizenship. Rubio later walked back his support for the bill, even though he still supports some path to legalization.

In an election year where candidates such as Cruz and Trump have surged in part because of their hard-line stances on immigration, Rubio is one of several candidates to shift positions on immigration reform. (Cruz even came under fire from opponents and scrutiny from fact-checkers recently for his claim in a December debate that he never supported legalization for undocumented immigrants.)

But for all the barbs back and forth, there's little actual chance of immigration reform policy moving in Congress. In the wake of congressional inaction (the compromise Rubio helped craft failed in the House in 2013), Obama has tried to go it alone by extending deportation relief to undocumented immigrants and some of their parents. His executive actions are held up in courts because of a lawsuit led by Cruz's home state of Texas. But it could set the stage for a Supreme Court decision on Obama's executive actions this summer -- months before the president leaves office, and months before Americans will decide who replaces him.

6. Supporting the eventual nominee

Even though host Fox Business is advertising a policy-focused night, politics and political process have to be a part of it this close to the actual voting.

The central storyline here is one that has dominated the Republican primary since this summer: In both polls and voter excitement, Trump is the undisputed front-runner.

(Philip Bump / The Washington Post)

But does the non-traditional candidate have the campaign infrastructure to get out voters on a cold, potentially snowy night?

Then there's the question of what support Trump would have if he were the nominee. By show of hands in the first GOP debate, the candidates all ostensibly pledged to support whomever would become the party's nominee -- all except Trump. (Fox News Channel's Bret Baier asked the candidates to raise their hands if they would not pledge to support the nominee and would not rule out a run as an independent. Trump was the only one with his hand raised, and either he or the question -- or maybe both -- were booed by the audience.)

But in the wake of Trump's suggestion to temporarily ban Muslims from the country, that has become a pledge many candidates appear to wish they could walk back -- especially as it looks increasingly possible that Trump could win the Republican nomination.

From former Florida governor Jeb Bush to Carly Fiorina, candidates are side-stepping questions lately about whether they'd support Trump if he were the nominee.