WASHINGTON -- Fox News announces Tuesday its decision on the 10 top polling Republican candidates -- out of a field that has mushroomed to 17 -- who will be invited for its first debate this week in the battle for the party's 2016 presidential nomination.

Fifteen months out from the election, the Republican race is dominating political coverage both for the extraordinary number of announced candidates and the fact that Donald Trump, the brash billionaire real estate baron, has taken a strong lead early in the competition despite stirring controversy by labeling Mexican immigrants as "criminals" and "rapists" and other remarks.

Initially, Trump was thought to have no chance in the race, but latest polls show him far outdistancing mainstream Republican candidates who had been expected to wage a three- or four-way battle for the nomination before Trump unexpectedly jumped into the contest in June. However, his negative numbers in the polls are also high and most observers are skeptical about his chances of securing the nomination.

An establishment Republican candidate was seen as emerging to compete in the general election against former Secretary of State Hillary Clinton, who was expected to coast to the Democratic nomination with little or no challenge.

But that race also has surprised political experts with the unexpected candidacy and strength of Vermont Senator Bernie Sanders, a self-described socialist who caucuses in Democrats.

And in recent days there were reports that Vice-President Joe Biden is seriously considering a late entry into the Democratic contest.

But nothing tops Trump's surprising captivation of Republican voters. Some polls show his backing has grown to nearly double that of former Florida Gov. Jeb Bush, once the front-runner.

Initially, Trump was thought to have no chance after announcing his candidacy with a speech that offended Mexican immigrants. He later debunked the hero status of Sen. John McCain, imprisoned and tortured for five years by North Vietnam.

Political observers had expected those comments to sink Trump's candidacy before it had really set sail. Instead he has struck a deep vein of discontent with conservative Republican voters.

Trump and two other Republicans were absent from the stage during a key forum in the state New Hampshire on Monday night when the rest of the field took part in a kind of warmup ahead of the Thursday debate. New Hampshire holds the first primary election next February in the state-by-state contests to choose the nominee.

The gathering offered a prime-time practice round for most of the would-be debaters, who addressed several contentious issues, immigration topping a list that also included abortion and climate change.

Without exception, the candidates on Monday aimed their criticism at Democrats instead of each other in a two-hour faceoff. Trump was not mentioned.

Fox has not said which polls it would use to determine the top 10 candidates. Several surveys have been released in recent days, with more expected Tuesday. The margins could make a difference for candidates separated by 1 or 2 points.

A Monmouth University survey released Monday found that only five candidates -- Trump, Bush, Wisconsin Gov. Scott Walker, Texas Sen. Ted Cruz, and former Arkansas Gov. Mike Huckabee -- are definitely in the top tier of candidates.