Trump had held the top spot in the previous 31 consecutive national polls, according to RealClearPolitics , posting a double-digit lead in all but four. In last month's NBC poll, Trump held a 13-point lead over Cruz.

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Cruz also leads Trump by 16 points in a head-to-head match-up.

The result is a major coup for Cruz on a day marked by deepening in tensions between the two leading GOP contenders as they prepare for Saturday's South Carolina primary.

The NBC/Wall Street Journal poll was conducted Feb. 14–16 after last Saturday's debate in South Carolina, where Trump attacked former President George W. Bush over the Iraq War and the 9/11 attacks.

It could suggest Trump's attacks backfired on him. The former president remains popular among many Republicans, and he has been campaigning with his brother Jeb in the Palmetto State.

But only time will tell whether the poll is a harbinger of things to come or merely an outlier.

Quinnipiac released a poll taken between Feb. 10 and 15 showing Trump with 39 percent and Cruz with 18 percent support. A USA Today poll taken Feb. 11 to 15 found Trump with 35 percent compared to 20 percent for Cruz.

Trump is also comfortably ahead in several recent polls in South Carolina.

Hours before the NBC/Wall Street Journal polling went public, Cruz bashed Trump during a press conference solely dedicated to pushing back at the real estate magnate's threat of legal action over a Cruz ad that casts Trump as pro-choice.

Cruz tried to call Trump's bluff by telling him to go forward with a lawsuit, while Trump responded with harsh statement chiding Cruz as "Teddy."

The numbers also spell bad news for Jeb Bush , who sits far behind the rest of the field, in sixth place with 4 percent. Bush battled Trump during Saturday's debate over the attacks on his brother's record.

The poll found Rubio to be the most palatable candidate to GOP voters, with 70 percent saying they could see themselves supporting him.

Cruz came in second in that category, with 65 percent. Carson came in third, with 62 percent, and Trump followed, with 56 percent. Both Kasich and Bush were under 50 percent.

The poll showed an increase in "very conservative" Republican voters from January's sample — a possible explanation for Trump's lower-than-expected numbers. When the February sample is weighed to match the January results, Trump leads Cruz by just 1 point.

NBC surveyed 400 registered Republican voters. The poll has a margin of error of 4.9 percentage points.