Donald Trump's once commanding lead over Ted Cruz in the fast-approaching South Carolina primary has now been reduced to just 5 points with less than 24 hours remaining before polls open.

A new NBC News/Wall Street Journal/Marist poll shows Trump leading Cruz 28 to 23 percent, with Florida Sen. Marco Rubio also still in striking distance at 15 percent. Former Florida Gov. Jeb Bush is the only other GOP candidate attracting double-digit support at 13 percent, while John Kasich and Ben Carson both clock in at 9 percent each.

The tightening numbers come just weeks after Trump led Cruz 36 to 20 percent in the same poll, and a series of other surveys also showed him with a strong double-digit cushion.

Cruz Ground Game Making a Difference?

Growing speculation is Cruz may have made up some of his once sizable deficit by turning to the same vaunted ground game he used to topple Trump in Iowa.

According to WFSA 12, at least four times as many GOP voters are expected to vote in the first Southern primary as in Iowa, but Cruz strategists are convinced the same "data-focused, volunteer-heavy strategy" could work wonders.

In all, some 10,000 Cruz volunteers have reportedly blanketed the state, recently prompted campaign spokesman Rick Tyler to boast "this is a for-real ground game. We're organized and we've got money."

On a regular basis, those entrusted with making phone calls on behalf of the campaign are prepped with talking points about Cruz's record, most of them stressing his conservative credentials. Armed with a smart phone app with information about targeted voters, other volunteers are counted on to knock on doors and further spread the word about their candidate.

"We're the ones that are making the connection, making it more personal," said Kristi Lisenbee, a 49-year-old campaign volunteer worker from Keller, Texas.

Researchers also found 71 percent of Trump's supporters insist they are "strongly" behind the real estate mogul, while 68 percent respond they feel the same way about Cruz and 52 percent do so for Rubio.

Meanwhile, Rubio is viewed as the top second choice of 24 percent of GOP voters, compared to Cruz and Trump at 17 and 11 percent, respectively.

On the Democratic side, the poll finds Hillary Clinton still leading Vermont Sen. Bernie Sanders by a staggering 28 points among likely voters.

Clinton Still Controls South Carolina

The former first lady now bags 60 percent of the vote, just four points down from her high of 64 percent last month, while Sanders still hovers at just 28 percent.

While Sanders leads Clinton among white voters, 51 to 46, her commanding 68 to 21 percent advantage among African Americans is more than enough to easily offset that small deficit.

Even among African Americans under the age of 45, Clinton is ahead of Sanders by 17 points, 52 percent to 35 percent.

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