Five things to watch for in tonight's GOP debate

Charleston, S.C. --  Most of the remaining candidates for the GOP nomination for president will gather here tonight for the second-to-last debate before the Feb. 1 Iowa caucuses.

We say "most," because Kentucky U.S. Sen. Rand Paul has decided to boycott the undercard debate, hosted by Fox Business News, after failing to make the main stage.

Regardless, here are five things to watch for in the main event, which begins at 9 p.m.:

1. When Trump gets asked about S.C. Gov. Nikki Haley, and he certainly should, does he go after her like he has every other critic? Haley, in giving the GOP response to the president's State of the Union address on Tuesday, said, "During anxious times, it can be tempting to follow the siren call of the angriest voices. We must resist that temptation." On Wednesday, she told several media outlets she was referring to Trump, but said it wasn't personal.

2. Do Trump and U.S. Sen. Ted Cruz of Texas finally take off the gloves and go after each other in public, face-to-face? For months, the two front-runners have danced a strange dance where no matter what Trump said, Cruz refused to criticize him. But, as Cruz has gained/taken the lead in Iowa polls, Trump has, well, trumpeted the question of whether Cruz's Canadian birth makes him eligible to be president. Cruz, again, has not taken the bait. 

3. Does Jeb Bush keep the heat on Marco Rubio? Bush has taken to focusing his attacks on his fellow Floridian, and not front-runners Cruz or Trump. A Bush super PAC, Right to Rise, has even bought ads criticizing Rubio's choice of footwear.

4. Can Rubio make a dent in the leaders? Rubio, trailing Cruz and Trump by 13 percentage points in Iowa, according to the Real Clear Politics polling average, appears to have plateaued . With two weeks to go before Iowa, he needs to find a message that works. Cruz's super PAC's hits on Rubio on immigration haven't helped.

5. Do the other candidates find traction at all? Also on stage, besides Trump, Cruz, Rubio and Bush, will be Ohio Gov. John Kasich, New Jersey Gov. Chris Christie and Dr. Ben Carson. All, as well as Bush, could be facing their final weeks on the campaign trail. After Iowa, the field figures to narrow further. Christie, at least, appears to be making a run in New Hampshire, meaning his campaign could extend beyond Feb. 1.

Bonus: Do high-profile area shootings become an issue? Tonight's debate takes place about three miles from where white North Charleston Police Officer Michael Slager allegedly shot Walter Scott, a black motorist, after Scott fled a traffic stop in April. Slager has been charged with murder. The arena is also about nine miles from where nine people were murdered at Mother Emanuel AME Church in Charleston. All of the dead were black. The alleged shooter, also charged with murder, is white.