Both Ben Carson and Donald Trump say they want to make America great again. But the Republican presidential contenders don’t always agree on how they’d do that.

Carson, the mild-mannered ex-neurosurgeon, and Trump, the bombastic businessman, are arguably the Republican Party’s men of the moment. Trump retains his No. 1 standing in national polls, and Carson is in second place, according to the latest RealClearPolitics average. On Monday, a Monmouth University poll showed the two White House hopefuls tied in Iowa, the first state to hold presidential caucuses.

The two share some of the same policy positions, including a disdain for President Barack Obama’s health-care law. But they differ in other ways that may serve to drive a wedge between them as Republicans choose their candidate.

Here’s one: a flat tax. Carson likes the idea. Trump says he’s got a problem with it. “It would have to be somewhere between 10 and 15%,” Carson told Fox Business in August. “Early on probably higher, over the course of time lower.”

Last week, Trump said he had an issue with the flat tax just moments after he said he was open to it.

“The one problem I have with the flat tax is that rich people are paying the same as people that are making very little money,” he said on Fox News. “And I think there should be a graduation of some kind. Because as you make a certain amount of money, I think you should have to graduate upward.”

Climate change is another issue that Carson and Trump talk differently about. Trump called global warming a “hoax” back in 2013.

Carson sounded more measured when he spoke on the subject in Iowa in May. “I’ll tell you what I think about climate change,” Carson said, according to the Des Moines Register. “The temperature’s either going to go up or down at any point in time, so it really is not a big deal. What is a big deal is that the environment is under our control. We do have a responsibility to pass it on to those behind us in at least as good a condition as we found it, hopefully an improved condition.”

On immigration, Trump has grabbed headlines with his plan to build a wall between the U.S. and Mexico and proposal to deport immigrants living illegally in the U.S.

Carson has called a border wall “a good start” and is against what’s known as birthright citizenship. “It doesn’t make any sense to me that people could come in here, have a baby and that baby becomes an American citizen,” he said in Phoenix last month.

But Carson is taking border security a step further, proposing drone strikes on “caves” used to transport people across the border into the U.S. Speaking on CNN on Aug. 23, Carson emphasized he isn’t talking about killing people with drones. But he said: “There are caves that they utilize. Those caves can be eliminated. There are a number of possibilities — that could be one of them.”