Republican presidential candidate Jeb Bush speaks at the RedState Gathering, Aug. 8, 2015, in Atlanta, Ga.
Photo by David Goldman/AP

5 storylines to watch during tonight’s GOP debate

Tonight’s Republican debate in Las Vegas – the last big GOP event before the before the Christmas/New Year’s holiday with less than seven weeks until Iowa – will feature at least five good storylines to watch.

  • Donald Trump vs. Ted Cruz: They’re your two leaders in the Republican race; they’re both from the insurgent wing in the party; and they’ve both been allies of sorts – until now. As NBC’s Kasie Hunt reported on “Nightly News” last night, Trump and Cruz have started their first back-and-forth of the campaign. But don’t be surprised if that spat stops tonight. Why? Cruz seems to have gone out of his way to back off, and Trump (for the first time) is getting attacked by Rush Limbaugh for going after Cruz.
  • Cruz vs. Marco Rubio: As we wrote yesterday, this could very well be tonight’s biggest clash – with Cruz hitting Rubio on his co-authorship of the “Gang of Eight” immigration reform bill, and with Rubio firing back on Cruz’s opposition to the NSA surveillance program.
  • Rise of terrorism as the country’s No. 1 issue: That’s the headline from our newNBC/WSJ poll, with 40% of Americans now saying that national security and terrorism is the top priority for the federal government – up 19 points from when this question was last asked in April. This the first presidential debate, on either side, since the San Bernardino shootings.
  • Chris Christie returns to the main debate stage: Who says that one you get demoted to the undercard debate, you never make it back? Well, Christie’s poll numbers have increased, especially in all-important New Hampshire.
  • Jeb Bush’s last stand? The fact that this storyline is largely being overlooked is a story in of itself. How bad have things gotten for Bush? Look no further than the new Washington Post/ABC poll, which popped overnight: Trump 38%, Cruz 14%, Ben Carson 11%, Rubio 10%, Bush 7%. Or yesterday’s Monmouth poll: Trump 41%, Cruz 14%, Rubio 10%, Carson 9%, Bush 3%. Or Sunday’s NBC/WSJ poll: Trump 27%, Cruz 22%, Rubio 15%, Carson 11%, Bush 7%.

The least effective TV ad campaign in presidential history? Indeed, Jeb Bush and his allies have now spent more than $35 million on TV ads since September – almost as much as the rest of the GOP field combined, according to ad-spending data from SMG Delta. They’ve spent $17 million in New Hampshire (but Bush is just in single digits there). They’ve spent $9 million in Iowa (yet Bush’s fav/unfav is upside down among GOP caucus-goers in the Des Moines Register/Bloomberg poll). And they’ve spent another $5 million in South Carolina. Bottom line: The more money he and his allies spend, the worse it’s gotten for the campaign. And Bush now finds himself at a crossroads. Does his Right to Rise Super PAC starting using the $25-$30 million it has reserved for future ad buys to go negative against the rest of the field? Or does it call off the dogs? After tonight’s debate, Jeb Bush definitely has some soul-searching to do.

A Tale of Two Parties: While our new NBC/WSJ poll shows that national security/terrorism has vaulted to become the country’s No. 1 issue, there is still a significant divide by party. According to the poll, 58% of Republicans say national security/terrorism is the top concern, versus 12% who say the economy/jobs. But among Democrats, it’s 33% economy/jobs, 26% national security/terrorism. Just something to consider as the Democratic and Republican presidential primaries continue to play out.

Hillary’s tough balancing act: In Minnesota today at 4:00 pm ET, Hillary Clinton will deliver remarks on how she would fight terrorism and radicalization at home. Clinton, according to an aide, “will propose a comprehensive strategy to counter each step in the process that can lead to a terrorist attack like San Bernardino, from recruitment, to training, to planning, to execution, all while staying true to our values.” Yet as one of us adds, “[T]he former secretary of state is walking a delicate balancing act between a Republican presidential field that is proposing an aggressive and sometimes controversial response to fight ISIS and the policies of President Barack Obama. Clinton’s four years at the top of the Obama foreign policy team has tied her to her former boss but he is losing trust among Americans that he can keep them safe.”

Ugly scene at last night’s Trump rally: Finally, don’t miss this piece fromMSNBC’s Benjy Sarlin: “Some Donald Trump supporters attending the Republican presidential candidate’s rally in Las Vegas on the eve of the next GOP debate treated protesters with unusual hatred Monday night. Trump was interrupted several times by Black Lives Matter protesters, prompting a furious reaction from the crowd. Some screamed, pointed, and at one point shoved a heckler as hotel security swooped in to remove them. As one man was dragged away, Trump supporters variously yelled, ‘Shoot him!’ ‘Kick his ass,’ and ‘Light the motherf—-r on fire!’ A large middle aged man shouted, ‘Sieg heil!’ — a Nazi Germany-era salute — as the protester was taken away.”

On the trail: Hillary Clinton gives her anti-radicalization speech at 4:00 pm ET in Minneapolis, MN… Rand Paul holds a pre-debate rally in Las Vegas at 2:30 pm ET… And Bernie Sanders campaigns in New Hampshire.

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This story originally appeared on NBCNews.com

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5 storylines to watch during tonight's GOP debate