But as Cruz celebrates a big win Tuesday in Wisconsin, a Cruz-Trump battle for a majority of delegates at July's Republican National Convention looks more and more likely. That means it's not out of the realm of possibility that Cruz could be the next GOP nominee.
Here are four reasons that should still worry Republicans -- plus one reason they'd probably prefer Cruz over Trump anyway:
1. Senate Republicans don't like Cruz
This first point is something many of us, whether you're a U.S. Senate candidate or not, can relate to: It's tough to work with a guy you don't like.
Cruz is one of the least popular members -- if not the least popular member -- of the Senate among both Democrats and Republicans. Two out of his 53 Senate GOP colleagues have endorsed him, and those two -- Sens. Mike Lee (Utah) and Lindsey O. Graham (S.C.) -- waited until March to do so. Graham made clear that he was holding his nose to support Cruz. He previously compared Trump and Cruz to being "poisoned" vs. being "shot."
It's easy to see why Senate Republicans are frustrated with him. Since running for Senate in 2012, Cruz has gone to war with his own party at least five high-profile times -- from engineering the opposition that led to a 2013 government shutdown in his attempt to kill the Affordable Care Act, to making his colleagues come back to work on a Saturday at the last minute in a futile attempt to stop President Obama's immigration actions from taking effect.
After all that drama, Senate Republicans might be in the unenviably awkward position this fall of having to publicly embrace their least favorite guy on campus. Perhaps Trump's rise has made them forget how little affection they have for Cruz; Cruz being the nominee could quickly reinforce that reality.
2. Cruz is broadly very unpopular, too
Here's what comes to mind for Senate Republicans -- and much of the nation -- when they think of Cruz: a socially conservative politician who caters to the religious and tea party right, and not many other people.
Republicans worry that Cruz's far-right politics may make it tough for him to appeal to a broader electorate, especially because he hasn't shown much of an inclination to support policies that might be more agreeable to moderate Republican and independent voters.
More than half of Americans -- 51 percent -- already think of Cruz unfavorably, according to a recent Washington Post-ABC News poll. That's nowhere near Trump's 67 percent unfavorability rating. But it is on par with that of Democratic front-runner Hillary Clinton, even as Cruz is less well-known. Cruz's favorable rating (35 percent), meanwhile, is 11 points lower than Clinton's (46 percent). He is not a popular guy.
Of course, we've never seen Cruz in a position where he has needed to appeal to moderate voters to win. He's a senator from Texas, one of the most conservative states in the nation, and he's running in one of the most conservative presidential primaries in recent history.
There's always the chance that if Cruz is nominated, he would step back from his proposals to build a wall along the Mexican border or patrol U.S. "Muslim neighborhoods" for suspected terrorists. But aside from offering some platitudes Tuesday night about "uniting" the party, he has not indicated that he would shift his policies.
3. His brand is harder to distance the party from
One silver lining for Republicans if Trump is their nominee is that the real estate magnate has a spotty conservative record at best. He likes the idea of universal health care and openly acknowledges that he's "new" to the antiabortion movement (a newness that contributed to him stepping in it on abortion recently).
Republicans can try to inoculate themselves from Trump by arguing that he's just not one of them. Yes, that's tricky to do when Trump is their presidential nominee, but they can contrast their conservative record with Trump's and argue that they don't agree with Trump's politics.
That story gets much harder for Republicans to tell if Cruz, whom no one would confuse for a Democrat, is their party's nominee.
Cruz could arguably make life harder for the handful of vulnerable Senate Republicans running for reelection in swingy or even blue states who don't want to be lumped in with such a conservative candidate. He's their colleague, after all, and many of them have joined in his quixotic pursuits.
4. Democrats think they have a case to make against Cruz
With Cruz's win in Wisconsin, Senate Democrats are taking a break from trying to link their Republican colleagues to Trump and test-driving what they'd say about Republicans if Cruz were their nominee.
Their first line of attack would be to punch through the party's veneer of unity by pulling comments from Senate Republicans predicting Cruz would be worse than Trump for "our chances of keeping the majority." Senate Democrats' campaign arm sent out a memo Tuesday with news clips saying as much.
They could also still conceivably run ads like this one, replacing the name "Trump" with "Cruz."
The implication is clear: Senate Democrats' game plan to try to make Senate Republicans as unpopular as their nominee doesn't just go away with Trump.
BUT. Trump is a complete wildcard; Cruz isn't.
We'll end on a positive note for Senate Republicans considering a Cruz nomination.
Aside from unannounced weekend work or the occasional government shutdown, they know what they're getting from Cruz. He is a politician who has demonstrated that he can stick to his principles and navigate politically sticky situations with as much deftness as the next candidate.
Republicans, by contrast, have no idea what they're getting with Trump. One day he's suggesting Muslims be banned from the country, the next he's retweeting a quote from Mussolini, the next his campaign manager is getting charged with battery for allegedly grabbing a reporter's arm and bruising her.
As our own Chris Cillizza wrote after the Mussolini retweet incident, Trump's unpredictability is more dangerous for Republicans than anything else this presidential campaign could throw at them.
At least with Cruz, they can formulate a plan. With Trump, all bets are off.
Ted Cruz exits the presidential race