The Republican presidential nomination race has settled into two distinct tiers. 

Four candidates -- Donald Trump, Ted Cruz, Marco Rubio, and Ben Carson -- command nearly three-quarters of the party’s support.  The rest are just trying to break through. 

Here’s how the race stands according to the latest Fox News national poll on the 2016 election:

Trump receives 34 percent among Republican primary voters, Cruz gets 20 percent, Rubio 11 percent, and Carson 8 percent. 

Two weeks ago, it was Trump 35 percent, Cruz 20 percent, Rubio 13 percent, and Carson 10 percent.

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Trump hit a record-high 39 percent support in December, and Rubio hit his high-water mark of 14 percent in mid-November.  Cruz’s current 20 percent is his highest, which he hit for the first time earlier this month.  Carson hit 23 percent in October when he briefly flirted with Trump for the top spot. 

Cruz tops the list as the “second choice” among Trump backers -- and vice versa. 

The new poll, released Friday, finds that Jeb Bush and John Kasich each receive 4 percent, Chris Christie 3 percent, Mike Huckabee and Rand Paul 2 percent, Carly Fiorina and Jim Gilmore tie at 1 percent support, and Rick Santorum receives less than one percent.

“One of the most notable things about the race is that none of the lower tier candidates has been able to get any oxygen," says Republican pollster Daron Shaw, who along with Democratic pollster Chris Anderson conducts the Fox News Poll.

"Carson, Rubio, and Cruz managed some separation last summer, but since then the candidates have been mostly smothered by Trump and coverage of Trump.”

The poll also asks GOP primary voters if there is any candidate they just couldn’t support against the Democrat in November.  Some 15 percent say they would refuse to vote for Trump and 10 percent feel that way about Bush.  Another 8 percent would stay home instead of voting for Christie, and 6 percent say the same about Carson, Cruz, and Paul. 

Recently Trump began questioning Cruz’s eligibility to be president -- because Cruz was born in Canada to an American-citizen mother. 

The Constitution says only “natural-born citizens” can serve as the nation’s chief executive.  While the U.S. Supreme Court has never ruled on the issue, Republican primary voters have an opinion: 76 percent feel Cruz is eligible to be president of the United States.  Fifteen percent feel he isn’t. 

Sixty-one percent of voters overall say Cruz is eligible, while 27 percent disagree. 

Almost all GOP voters say the citizenship question won’t matter to their vote (88 percent).  For 10 percent, birther concerns will make them less likely to support Cruz.

Betrayal, dissatisfaction and anger are what’s mainly driving Republican voters.

Fifty-two percent of GOP primary voters feel “betrayed” by politicians in their party.  While that’s down from 62 percent in September, it’s far more than the 37 percent of Democratic primary voters who feel betrayed. 

A whopping 88 percent of Republican voters describe themselves as dissatisfied with the way the federal government is working.  That includes 40 percent who are flat out angry.

In the January 14 GOP debate on the Fox Business Network, Trump said, “I’m very, very angry … I’m angry because our country is a mess.”

“Angry” Republican voters go for Trump over Cruz by a 49-22 percent margin. 

Among those who are just “dissatisfied” and not angry, Trump (24 percent) and Cruz (20 percent) are more evenly matched. 

Nearly three-quarters of voters overall are unhappy with Uncle Sam: 47 percent are dissatisfied with the way the federal government is working and 27 percent say they are angry.

GOP primary voters want a candidate who is a strong leader (24 percent) and is “willing to tell it like it is” (23 percent).  Almost as many want someone who has true conservative values (19 percent).  Far fewer say having the right experience (8 percent) or winning in November (7 percent) are top of mind for them when picking a candidate.

The Fox News poll is based on landline and cellphone interviews with 1,009 randomly chosen registered voters nationwide and was conducted under the joint direction of Anderson Robbins Research (D) and Shaw & Company Research (R) from January 18-21, 2016. The poll has a margin of sampling error of plus or minus three percentage points for all registered voters, and 4.5 points for the Republican primary voter sample (405).