Republican presidential candidate Sen. Marco Rubio, R-Fla, speaks at a rally in Reno, Nevada, Feb. 22, 2016.
Republican presidential candidate Sen. Marco Rubio, R-Fla, speaks at a rally in Reno, Nevada, Feb. 22, 2016.

After winning primaries in the northeast and southeast, billionaire Donald Trump looks poised to get a win in the western part of the United States in Tuesday's Nevada caucus.

The latest polls show Trump leading his Republican challengers with about 40 percent support, while Senators Marco Rubio and Ted Cruz remained locked in another tight battle for second place with about 20 percent each.

A good showing in Nevada will give candidates momentum heading into next week's so-called Super Tuesday when a dozen states will vote on the same day.  At that point, one-third of states will have held their nominating contests.

Rubio, who unlike Cruz and Trump has not yet won a state, picked up endorsements from veteran Republicans ahead of the Nevada caucus, including longtime senator and former presidential candidate Bob Dole.

U.S. Republican presidential candidate Ted Cruz sp
U.S. Republican presidential candidate Ted Cruz speaks with the media before a campaign rally in Las Vegas, Nevada, Feb. 22, 2016.


Meanwhile, Cruz fired his campaign spokesman for social media posts involving a video that falsely alleged Rubio insulted the Bible.

Trump leads, some Republicans skeptical

Trump has won two states in a row, after taking the primaries in New Hampshire and South Carolina.  With those wins and a double-digit lead in national polls, the businessman who has never been elected to public office became the clear leader for the Republican presidential nomination.

But surveys show a sizeable group of Republican voters remain opposed to his candidacy, either because they do not think he can win the White House in November's national election against the leading Democratic contender, former Secretary of State Hillary Clinton, or because they do not believe his proclaimed support for a variety of conservative positions that most U.S. Republicans adhere to.

Republican U.S. presidential candidate Donald Trum
Republican U.S. presidential candidate Donald Trump speaks at a campaign rally at the Georgia World Congress Center in Atlanta, Georgia, Feb. 21, 2016.

One prominent conservative pundit, Erick Erickson, said Monday he would never vote for Trump if he is the Republican nominee.  Years ago, Trump said he strongly supported abortion rights in the United States, but now tells voters he is pro-life, favoring a curb on the right of American women to the medical procedure.

But Erickson said he has become convinced that Trump's transformation on the issue "is a conversion of convenience."  A one-time local Republican official, Erickson said the party "will not deserve my support and will not get it if it chooses to nominate a pro-abortion liberal masquerading as a conservative."

Democratic presidential candidate Hillary Clinton
Democratic presidential candidate Hillary Clinton greets supporters with her husband, former President Bill Clinton, at a Nevada Democratic caucus rally in Las Vegas, Feb. 20, 2016.

South Carolina primary

The next Democratic presidential nominating contest is in South Carolina on Saturday. 

It is a party primary election that Clinton, the country's top diplomat from 2009 to 2013, is heavily favored to win over her remaining rival, Vermont Senator Bernie Sanders, a democratic socialist who has focused his campaign against the growing income inequality in the United States and the power of Wall Street financial titans.

WATCH: Trump, Cruz supporters in Nevada