Donald Trump Donald John TrumpHR McMaster says president's policy to withdraw troops from Afghanistan is 'unwise' Cast of 'Parks and Rec' reunite for virtual town hall to address Wisconsin voters Biden says Trump should step down over coronavirus response MORE and Texas Sen. Ted Cruz Rafael (Ted) Edward CruzLoeffler calls for hearing in wake of Netflix's 'Cuties' Health care in the crosshairs with new Trump Supreme Court list 'Parks and Rec' cast members hosting special reunion to raise money for Wisconsin Democrats MORE are circling each other in the race for the Republican nomination and appear to be on the brink of an epic clash.

While the candidates have for months observed a kind of truce, party insiders believe that’s about to change — and many are speculating about just how explosive things will get.

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Trump appears to be feeling the heat from Cruz, who now enjoys a small but significant polling lead in Iowa, which will hold the party's first nominating contest on Feb. 1.

In recent days, he has begun to jab Cruz in more aggressive ways, questioning the senator’s eligibility for president and attacking his position on immigration reform.

Mitchell McKinney, a professor of political communication at the University of Missouri, speculated that recent comments from Trump are being made with the thought that, “as we’re heading into Iowa, I’ll give the Cruz supporters something to doubt; maybe something to second-guess their choice.”

Trump earlier this week raised the issue of Cruz’s Canadian birth, suggesting that the Texas senator could face a lengthy court battle over whether he is constitutionally qualified for the White House. Most experts doubt that Cruz would confront any serious peril in this regard, given that his mother is a U.S.-born citizen.

Cruz’s initial response was a tweet implying that Trump had “jumped the shark” in the infamous manner of the “Happy Days” TV show.

But by Thursday, the senator’s wife, Heidi, was taking a more serious tack, insisting her husband is “indisputably a U.S. citizen.” Cruz himself, in an interview with Bloomberg, insisted that “the legal question is straightforward and clear” regarding his eligibility.

Trump had presented his statements about Cruz’s birthplace carefully, suggesting he was simply raising the possibility of Democrats making legal trouble for Cruz. But Trump broadened his attack on Wednesday when he said during an interview with CNN’s Wolf Blitzer that Cruz was “in favor of amnesty” for illegal immigrants.

Trump’s new tone is still not as sharp as some people expected, however, given how serious a threat they believe Cruz poses for his candidacy.

“It’s puzzling why Trump is not going after Cruz more directly, because Cruz is a bigger threat than Trump thinks he is,” said GOP strategist Matt Mackowiak. “If Trump loses Iowa by a significant margin and then somehow loses New Hampshire … I think the whole thing falls apart. The piercing of that invincibility begins in Iowa.”

Trump has thrown jabs at Cruz in the past, but they have lacked the snap of his verbal punches at the rest of the field. The most memorable — a suggestion that Cruz had behaved “like a little bit of a maniac” in the Senate — generated a backlash among conservative talk-radio figures who had previously been supportive of Trump.

The mini-controversy also produced one of the rare moments when Trump backed away from his earlier rhetoric. Asked about his "maniac" comment during the most recent Republican debate, held in Las Vegas last month, Trump said, “I have gotten to know [Cruz] over the last three or four days. He has a wonderful temperament. He’s just fine. Don’t worry about it.”

The reluctance to attack has seemed to run both ways. At the same debate, Cruz was asked about Trump’s controversial proposal to bar Muslims from entering the United States for a limited, though unspecified, time.

“I understand why Donald made that proposal,” Cruz said, adding that he himself advocated a “more narrowly focused” approach.

That was only the most recent example of Cruz’s propensity to stay on Trump’s good side. Last month, he tweeted that he and Trump getting into a “cage match” was “the Establishment’s only hope,” adding for good measure, “@realDonaldTrump is terrific.”

Trump and Cruz also appeared side by side in September at a rally on Capitol Hill against President Obama’s nuclear deal with Iran.

Several unaligned Republican strategists have long believed that Cruz is positioning himself as the alternative choice for Trump voters if the real estate magnate stumbles.

“A number of candidates operate on the assumption that Trump won’t get to there from here — that the nomination will be a bridge too far,” said John McKager "Mac" Stipanovich, a longtime Republican operative in Florida. “That being the case, the people who support Trump will either go home or they will need a new horse to ride. Clearly, Cruz intends to be that horse.”

The obvious weak link in that strategy, some suggest, is that it assumes Trump’s candidacy will implode — something that has been predicted since his campaign’s inception but hasn’t happened.

As of Thursday evening, Trump held a 15-point lead in the RealClearPolitics national polling average and a two-to-one advantage over his closest rival in New Hampshire, which holds its primary eight days after the Iowa caucuses.

Referring to Cruz’s presumed expectation that Trump will fade, Mackowiak, the GOP strategist, said, “It could go the way he wants it to. But it could also go the other way, where Trump is too strong and is on a glide path to the nomination.”

Others praise Cruz for avoiding conflict with the combative Trump.

Dave Carney, an unaligned GOP strategist in New Hampshire, ran through a list of other GOP contenders who had attacked Trump, including ex-candidates such as former Texas Gov. Rick Perry and South Carolina Sen. Lindsey Graham Lindsey Olin GrahamThe Hill's Campaign Report: Arizona shifts towards Biden | Biden prepares for drive-in town hall | New Biden ad targets Latino voters Senate Democrats' campaign arm announces seven-figure investment to boost Graham challenger Graham: Comey to testify about FBI's Russia probe, Mueller declined invitation MORE, as well as those still running, such as former Florida Gov. Jeb Bush.

“I don’t think it’s gotten any of them any mileage,” Carney said. “In a multi-candidate field, when you are talking about anything other than your own message, it’s a total distraction. Cruz has taken an alternative path, and I think that’s because their campaign is thought-out, strategically, over the long haul.”

But Carney also suggested that, with Cruz becoming a definite favorite to win Iowa, Trump’s relative civility toward him is coming to an end.

“The only person who really stands in the way of Trump is Cruz,” he said. “Trump is going to realize that and unload on him.”