Donald Trump, a candidate even Republicans once considered a side show, increases his lead yet again in the nomination race, according to the latest Fox News national poll.

The poll also finds Ted Cruz ticking up, Marco Rubio slipping, and Ben Carson dropping.

Trump hits a high of 39 percent among Republican primary voters, up from 28 percent a month ago. The increase comes mainly from men, white evangelical Christians, and voters without a college degree -- and at the expense of Carson.

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However, the poll is not all good news for the Donald. Despite his increasing advantage in the primary, his support in the general election is down. More on that later.

All interviews for the new poll were done since Tuesday’s GOP debate in Las Vegas.

Trump’s 11-point jump in support comes after his comments about stopping non-U.S. Muslims from coming into the country.

Seven in 10 GOP primary voters are in favor of a temporary ban.

Back in June, the Fox News poll asked whether voters felt the real-estate mogul and reality TV star was a serious candidate or a “side show.” At that time, 64 percent of GOP primary voters said Trump was a side show, rather than a serious candidate (29 percent). Today, it’s the reverse -- 33 percent of GOP voters say he’s a side show and 65 percent say serious candidate.

Support for Trump in the GOP race is more than double that for Cruz, who comes in second with 18 percent. That’s up four points since mid-November and nearly twice the 10 percent he had in October.

Rubio garners 11 percent, down three points from 14 percent last month.

Carson drops to 9 percent. He was at 18 percent last month and had a high of 23 percent support earlier this fall.

After that, Jeb Bush, Chris Christie, Carly Fiorina, and Rand Paul get three percent each. All others receive two percent or less.

The favorites among white evangelical Christians are Trump (39 percent), Cruz (26 percent) and Carson (12 percent). Since last month, Trump is up 14 points among this group, Cruz is up 8 and Carson is down 12.

Trump’s backing among men went from 28 percent in November to 46 percent now. (He mostly held steady among women at 30 percent.)

Forty-five percent of GOP primary voters without a college degree go for Trump -- up from 30 percent in November. At the same time support for Carson fell 9 points among this group.

For comparison, support among those with a college degree is more mixed: Trump (31 percent), Cruz (19 percent), Rubio (16 percent) and Carson (10 percent).

Is Trump hitting his ceiling? He only gains two points when the race is narrowed to just the top four candidates. That said, Trump still has a significant advantage in this quartet: he captures 41 percent of GOP primary voters, followed by Cruz at 25 percent, Rubio at 17 percent and Carson at 12 percent.

By wide margins, Trump is seen as the candidate most qualified to handle the economy (52 percent) and most effective against the Islamic extremist group ISIS (49 percent). Cruz comes in a distant second on both (13 percent economy, 17 percent ISIS) -- and no other candidate gets into double-digits.

Views are more mixed when primary voters are asked who is most qualified to be commander-in-chief. Trump is the top pick at 31 percent, followed by Cruz at 21 percent. Next it’s Rubio at 11 percent and Bush at 8 percent. Trump (+12 points) and Cruz (+8 points) both improved on this measure since early November. Only six percent see Carson as most qualified to be commander-in-chief now -- an 11-point drop since last month.

Despite Trump’s strong showing among primary voters, other Republicans perform better when all registered voters are asked their preference in hypothetical general election ballot tests.

When matched against presumptive Democratic nominee Hillary Clinton, Rubio has a two-point edge (45 percent vs. 43 percent), and Cruz and Clinton tie (45 percent). Carson is down by two.

Clinton now overtakes Trump: 49 percent vs. 38 percent. Her 11-point advantage is outside the poll’s three-point error margin. A month ago, Trump was up by 5 points (November 16-19).

“Trump has helped himself and Democrats over the last month,” says Democratic pollster Chris Anderson. “And while the outside probability of a Trump presidency is enough to make many Democrats root against him in the primary, they can take some solace in that Trump is clearly the least electable of the leading Republican candidates.”

Rubio does a better job retaining the party faithful than Trump in these matchups: 85 percent of GOP voters would back Rubio over Clinton, while 75 percent would go for Trump over the Democrat.

Another reason for Rubio's stronger showing: younger voters.

"Clinton tops Trump by 29 points among those under 30, while Rubio bests her by 4 points," notes Republican pollster Daron Shaw, who conducts the Fox News Poll along with Anderson. "For whatever reason, millennials are more resistant to Trump's appeal."

Trump’s weakness against Clinton is not surprising given nearly 6 in 10 registered voters don’t take his candidacy seriously: 57 percent say he’s a side show, while far fewer, 39 percent, think he’s a serious candidate.

Even so, that’s an improvement, as 77 percent called him a side show in June.

There’s been little movement on the Democratic side. By a 56-34 percent margin, Clinton tops Bernie Sanders among Democratic primary voters. Last month it was 55-32 percent.

Sanders supporters (61 percent) are more enthusiastic than Clinton supporters (52 percent) about their choice of candidates this year.

The Fox News poll is based on landline and cellphone interviews with 1,013 randomly chosen registered voters nationwide and was conducted under the joint direction of Anderson Robbins Research (D) and Shaw & Company Research (R) from December 16-17, 2015. The poll has a margin of sampling error of plus or minus three percentage points for all registered voters, and five points for both the Democratic (390) and Republican (402) primary voter samples.