As recently as early December, Ted Cruz held a 10-point lead over Donald Trump in Iowa. 

But it all started to go downhill for the Texas senator when Donald Trump began raising the birther issue.  Trump parlayed a dubious argument about Cruz's eligibility to serve as president into an effective attack that raised doubts about the candidate.  Those doubts translated into sinking poll numbers until now, the day before the caucuses, where Trump has vaulted back into the lead.

To make matters worse, Cruz sent out an ill-advised mailer that some believe was trying to shame Iowans into attending the caucuses.  Whatever its intent, the mailer is not helping Cruz to gather momentum going into tonight's contest.

Weekly Standard:

Cruz has spent the last several days defending himself on all sides, from Trump and Marco Rubio to the pro-ethanol lobby and GOP governor Terry Branstad. He's found himself looking for support in small, rural corners of the state that ought to be already solidly in his corner. He's been forced to defend a shady last-minute get-out-the-vote effort. And in the Iowa Poll, he's seen his favorability dip by 11 points since the beginning of January.

So Cruz's final pitch in Sioux City, aided by endorsements celebrity and otherwise, didn't seem like the culmination of a campaign that aimed to consolidate the conservative vote. It felt more like a rag-tag effort, patchworked together to stave off an out-of-control revolution that threatens the Republican party, or at least Cruz's role as its leading conservative firebrand.

"Rag-tag" might very well describe what propelled the past two Republican victors in Iowa, Rick Santorum and Mike Huckabee, and it was reasonable to expect Cruz to expand on what they built. After all, Santorum and Huckabee were relatively unknown when they won Iowa. Huckabee was a small-state governor with a hillbilly name, and in Santorum's last race in 2006, he lost his reelection bid by 18 points. Their victories here were surprising because they were genuine underdogs.

It's never a good sign when a candidate is playing defense in the waning hours of a campaign, defending his own turf rather than trying to poach votes from the other candidates.  But the Iowa caucuses offer Cruz a second chance of sorts – one that could salvage his flagging campaign and re-establish the momentum he's lost.

The latest Des Moines Register poll shows a hidden advantage for Cruz.  While he trails Trump by 5 points, only Marco Rubio has more "second choice" support than Cruz.  Combined first- and second-choice support gives Cruz 40% of the Iowa vote to Trump's 35%.  Since about 40% of Iowans were still undecided going into this weekend, a lot could happen to flip the poll results in Cruz's favor.

So when the night is over and the votes counted, it is possible to see a Cruz victory.  If not, the candidate will have to find a way to regain momentum going into New Hampshire, or his candidacy will be in serious trouble.