WEST DES MOINES, Iowa — Sen. Ben Sasse on Wednesday joined the 2016 campaign to stump for Marco Rubio, in another sign that the conservative movement is coalescing to stop Republican presidential frontrunner Donald Trump.

The Nebraska Republican isn't endorsing Rubio, his Senate colleague from Florida. Indeed, Sasse, 43, also plans to hit the trail in Iowa for Texas Sen. Ted Cruz and businesswoman Carly Fiorina ahead of first votes in Monday's caucuses. But Sasse's appearance on behalf of Cruz, Fiorina and Rubio amounts to another (unusual) endorsement of sorts: Anybody but Trump.

Sasse is a constitutionalist who advanced to the Senate in 2014 after winning over conservative voters in his competitive primary election. His decision to join the Republican primary campaign in an explicit bid to help halt Trump's momentum reflects a deep anxiety among free-market, limited government conservative activists. They view the New York celebrity businessman as another President Obama.

Just like Obama, they contend, Trump supports big government and would illegally push and the Constitutional boundaries of executive authority if he didn't get what he wanted from Congress. In an interview with the Washington Examiner just after introducing Rubio to a packed rally of 325 supporters, Sasse, decked out in Nebraska Cornhusker red, discussed his concerns about Trump and why he scares conservatives.

Examiner: Why did you decide to get off the mat and campaign against Trump?

Sasse: We already one post-constitutional party in this country, and my party shouldn't go down that same path. We have a frontrunner right now who literally says thinks like: When I'm elected president, I'll be able to do whatever I want to do. We should unpack would that means. President Obama was elected largely without being vetted in 2008. We shouldn't' have a 2016 election where one of the major party nominee's views on core issues, like what he thinks about the Constitution are not fully explored.

This is a guy who, when asked what he thinks of President Obama's executive orders, he says, well, maybe he's paved a new way. The system of government we have, has the legislature pass laws and the executive is supposed to faithfully execute them. The executive's not a king.

Examiner: Why didn't the conservative movement speak out sooner about their concerns?

Sasse: I am not opposed to President Obama's unilateralism because he's a Democrat; I'm opposed to it because it violates his oath. I said [on the Senate floor] that I would oppose any future Republican presidents who acted like Obama. As this campaign has unfolded and election begins next week, Iowans need to have their minds focused on this really important distinction:

Trump has done a good job of articulating much of what's wrong in this country. There's a lot wrong. He's a really smart guy. I don't think the media gives him nearly enough credit. He's a really, really smart guy, who is clearly strategic and is good at speaking bluntly about a lot of things that need to be spoken bluntly [about.] But there's a difference between diagnosis and proposed solution. And what we do to make America great again is unite around the things that bring us together, like a belief in limited government.

We don't unite around the ego of one guy.

Examiner: Did conservatives wait too long to speak up?

Sasse: Not a vote has been cast yet. The voters in Iowa get to go first and they start on Monday. No. 2, I made a speech on the Senate floor in December, called the thought experiment about a President Donald Trump. So, this isn't new to me this week, I've been talking about this for two months. Third, I was in New Hampshire last weekend, and the things I heard on the ground in New Hampshire are exactly the debates that I hear in the grocery story in my town of Freemont, Neb. Which is, people praising Trump's diagnosis, and saying: But who the hell is he and what does he believe?

He says he hates the concept of guns, he says he's for single payer universal healthcare, he says he's for a $6 trillion tax increase? Well I guess these aren't his positions anymore. But when he was thinking about running as a Democrat, these were his positions. Where has he ever explained what his actual views are on these issues and what is his view of the U.S. Constitution? Because that is your job — to defend the U.S. Constitution. I haven't heard that. And, I think the one thing the media misses, is on the ground in states like this, and my state, and what I heard last weekend in New Hampshire, is people are worried that Trump being good at diagnosis is not the same thing as Trump being the solution for that which ails us.

Examiner: A lot of your Republican colleagues in Congress say they would choose Trump over Cruz if those were their only choices. What are they missing?

Sasse: What I think is missing is the media that keeps asking student council race questions. Because I don't ever hear these questions from people on the ground in Nebraska. What I hear is, who's going to make America great again? And the way to make America great again is a constitutional recovery, not a Republican version of Barack Obama.

This interview transcript was edited for length and clarity.