Story highlights The race for first place in Iowa's Republican caucuses appears to be shaping into a two-man sprint between businessman Donald Trump and Texas Sen. Ted Cruz

It sets up a showdown before the nation's first presidential contest on Monday

Marshalltown, Iowa (CNN) The race for first place in Iowa's Republican caucuses appears to be shaping into a two-man sprint between businessman Donald Trump and Texas Sen. Ted Cruz, setting up a showdown before the nation's first presidential contest on Monday.

"This has now become a binary decision," Iowa Rep. Steve King, who has endorsed Cruz, told a gathering of Cruz supporters Tuesday. "Either you're for Trump, or your vote needs to be Cruz."

Until recently, Cruz and Trump spent most of campaign season amiably in the race, but the two have become increasingly critical of each other. Trump has called Cruz "a nasty guy" and Cruz questioned Trump's ability to serve in office after the billionaire businessman dropped out of this week's presidential debate over a scuffle with Fox News.

Trump and Cruz have campaigned on the same side of most issues, so for many caucus-goers, their choice comes down to personality.

For many Cruz supporters who spoke to CNN, Trump is too much of a "showman," who they fear lacks the skills to follow through on his promises if elected. And despite Trump's recent rhetoric about embracing religion, some Christians here struggle to see how Trump's past works match his professions of faith.

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