Donald Trump. AP Chris Christie and Donald Trump seem, in some ways, like a match made in heaven. In others, like on a variety of policy areas, it's hard to square Christie's surprise endorsement last week for his one-time presidential rival.

But what was most telling about Christie's endorsement was that it came with a rising line of thinking within the Republican Party, that Trump is the most electable candidate the GOP has left to face off against Hillary Clinton.

"The single most important thing for the Republican Party is to nominate the person who gives us the best chance to beat Hillary Clinton," Christie said, appearing with Trump at a press conference before a rally in Texas.

"I can guarantee you that the one person that Hillary and Bill Clinton do not want to see on that stage come next September is Donald Trump," the New Jersey governor continued.

That is becoming clear to a number of Republican and even Democratic observers, who are starting to come to grips with the coming months of a general election that could feature a Trump-Clinton showdown for the ages.

Democrats privately, and even publicly, have reacted with almost universal glee at a possible Trump nomination, pointing to his bombastic and nativist rhetoric they believe would alienate wide swaths of the American public.

But here's why many Republican and Democratic observers say Trump can win: He has inspired a flood of enthusiasm that has led to record Republican turnout in each of the four states that have so far voted. He also has crossover appeal that one GOP consultant compared to Ronald Reagan, and the potential to tap in to typically unheard-of constituencies for the GOP, something that could elude other candidates like Sens. Marco Rubio and Ted Cruz, as well as Ohio Gov. John Kasich.

And he has certainly proven his credentials by navigating his way to a dominating lead in a once 17-member Republican presidential field by assembling of one of the most broad-based coalitions in recent Republican Party history.

"They do not know the playbook with Donald Trump because he is rewriting the playbook," Christie said Friday of the Clintons. "He is rewriting the playbook of American politics."

Christie introduces Trump at a rally. AP Photo/Andrew Harnik

In the Nevada caucuses last Tuesday, 34,531 people came out to vote for Trump. In 2012, 32,894 people came out to caucus in the state — for all of the Republican candidates that year.

The pattern has been the same in the other three GOP primaries that have weighed in so far/ In Iowa, Republican turnout beat its previous record by about 50%. In New Hampshire, more than 280,000 GOP voters shattered their party's previous turnout record. And in South Carolina, more than 730,000 people showed up to the polls.

"Trump has huge turnout potential," said one veteran GOP operative who worked on a 2016 campaign. The operative said one could "certainly make that case" that Trump could be the strongest potential Republican nominee.

The Democratic side, as many Republicans have been quick to note, has not seen the same enthusiasm thus far. Turnout has been down — by significant margins — from 2008, when Clinton first ran for president against Barack Obama.

"I find it's a little bit of déjà vu all over again," said Doug Watts, a former senior adviser to one of Trump's Republican rivals, retired neurosurgeon Ben Carson.

By déjà vu Watts was referring to the candidacy of none other than the man who is perhaps most often invoked in Republican politics: Ronald Reagan.

Watts worked on the advertising team of Reagan's 1984 reelection campaign, and he was a keen observer four years earlier. Indeed, he saw many parallels between Reagan and Trump — and not just their celebrity backgrounds.

"Look back to the early Reagan days," Watts told Business Insider. "They said, 'Oh, that Ronald Reagan. He just shoots from the hip. He can't bring anyone together. He can't actually win.'"

"He showed them," Watts said.

"Let's make America great again." (AP Photo)

Polls, which at this point remain an unreliable predictor of the race to come, have thus far shown mixed results for Trump in a potential head-to-head matchup against Clinton.

Recent Fox News and Quinnipiac University polls, for example, found that Trump would trail Clinton by as many as five points nationally at this point, while Cruz, Rubio, and Kasich narrowly edged the former secretary of state. A USA Today/Suffolk University poll released the day before, however, found him beating Clinton — though the survey also showed Rubio and Kasich with larger leads.

In any case, Trump's three-point average disadvantage against Clinton, at this point, is a far cry from where he stood last June, when he entered the race. A CNN poll conducted two weeks after his presidential announcement found that Clinton would trounce him by 24 points.

"Presidential campaigns rely on precision and predictability," said Ben LaBolt, a former top adviser to Obama's campaigns.

He added: "You've got to develop a model that predicts voter turnout on both sides. Trump is such an unconventional candidate it'll be hard to accurately predict who is going to show up for him — HIS campaign doesn't even know."

However, Trump could have an unexpected problem area: Republicans.

Some influential conservatives with devoted followings have become so vehemently opposed to his candidacy that they say they will actively work against him in a general election. Erick Erickson, the conservative pundit and talk-show host, wrote last week that he would never vote for Trump. Similarly, a #NeverTrump hashtag popped up on social media last weekend that featured many prominent Republican and conservative voices.

Tom Jensen, the director of Public Policy Polling, told Business Insider that his firm has found that Trump receives only 77% of votes from Republicans who cast ballots for 2012 nominee Mitt Romney. That compares with 85% of those Romney voters who would back both Rubio and Cruz.

"So he clearly is dealing with an issue where a lot of normal Republican voters are hesitant to say they would get behind him," Jensen said. "Maybe they all will in the end and if that’s the case maybe he would in the end be stronger than Rubio — time will tell!"

Trump and Marco Rubio. REUTERS/Jonathan Ernst

Other pollsters and strategists suggest that Trump, more than any other GOP candidate, would be a potential godsend for Democratic turnout efforts.

If Clinton isn't an inspiring candidate, they say, the possibility of Trump in the White House would crystallize for many voters the stakes of the election. Clinton is counting on the type of strong support from young voters and minority groups like Latinos and African-Americans.

"On the one hand, Trump uses extreme rhetoric that could alienate independents and drive up the gender gap," LaBolt said.

Trump would likely argue against these theories.

He has frequently boasted about his potential ability to lure away Democratic and unaffiliated voters. Trump has promised not to cut Social Security, Medicare, and Medicaid. Several of his policy platforms break with the Republican Party's conservative elite. And he has shown the unique ability to tap into voters' frustration and anxiety.

In a November interview with Business Insider, he claimed that a "major Democrat" had told him that if any Republican were to win the election, he hoped it would be Trump.

"Now, I fully understood what he was saying. Very few other people did. But he said it, top of the line guy: 'If anybody had to win, I hope it's Donald Trump. Because he's the most reasonable guy,'" Trump said. "And I believe he's right. I will be able to make more deals, better deals than anybody else."

The New York Times last month investigated Trump's potential support with one of the most reliable Democratic constituencies: public-sector unions. Aside from his economic message, Trump's hard-line position on trade — he vehemently opposes the Trans-Pacific Partnership (TPP) deal negotiated by the Obama administration — aligns with that of unions.

The Service Employees International Union, which endorsed Clinton late last year, has a membership of more than 2 million workers. The union's president, Mary Kay Henry, told the paper that some of those members could be drawn to Trump. Other union leaders acknowledged it would be difficult to keep many of their members in line.

That would not be the case with a Rubio, Cruz, or Kasich as the nominee, said Watts, the former adviser to Carson.

"If you look at a Hillary Clinton vs. Trump in Michigan, Ohio, Pennsylvania, she's going to be talking about the message of breaking the glass ceiling while he's telling people what they want to hear," Watts said.

Donald Trump. REUTERS/David Mdzinarishvili

Trump and Clinton have offered the electorate a preview of their general-election battle — and it has energized Republicans who have long yearned for a more direct confrontation of the Democratic Party's nominees and their elites.

It came in December, when Clinton accused Trump of having a "penchant" for sexism. Trump used that remark to launch weeks of attacks on Clinton and her husband, former President Bill Clinton. His attacks helped fuel the resurgence of the press' new explorations of Clinton's sexual trysts. Trump said Bill Clinton was "one of the great women abusers of all time."

This was an example of Trump's "secret sauce," as Watts called it. He figures out a rival's weakness and exploits it. Along the way, he throws away supposed "political correctness" — he holds almost nothing back in his attacks.

Watts explained how Clinton might view a general-election matchup against Trump: "He let the dog out of the pound once. If I raise that again, I'm going to have a pack of dogs after me."

Voters were offered an implicit contrast last week by none other than Mitt Romney, the 2012 nominee. Romney threw out the possibility that there could be a "bombshell" lurking in Trump's tax returns, urging him and other candidates to release their tax information. The next day, after Trump unloaded on him, Romney offered that Trump might be "scared."

To many Republicans, it marked some of Romney's toughest talk, and it's the kind of tactics for which many of them longed when Romney was facing off against Obama.

As one Republican strategist unaffiliated with either Trump or Romney quipped to Business Insider: "If he had done this in 2012, then maybe he'd be running for reelection."