As presidential politics goes, most would agree it's been a roller coaster of a year filled with surprises that no one predicted when 2015 began.
Here are five of 2015's big political moments...
The ones who didn't run
Mitt Romney, the 2012 Republican nominee, looked hard at a third White House bid, and although he concluded he could win the nomination, he wasn't certain he could win the presidency. So, he stepped aside for "one of our next generation of Republican leaders, one who may not be as well known as I am today,...one who is just getting started."
Had he run, it likely wouldn't have stopped Trump, who thought a Romney 2016 run was a bad idea. "He had his chance and he blew it," he told Breitbart News.
Joe Biden also wanted to run, and he would have entered the race, had it not been for the death of his son, Beau Biden, in May.
The vice president struggled with the decision for months before dropping the idea. "If I thought we could've put together the campaign that our supporters deserved, that our contributors deserved, I would've done it," he told CBS News' Norah O'Donnell.
...And the one who did run
A year ago, there weren't many pundits predicting that Donald Trump would run for president, much less dominate the race in the way he has. He had flirted with a run in 1988, 2000 and 2012 -- and demurred each time. But now, he's in, and he has upended the race. Trump has dominated traditional and social media coverage with his tweet-rants, topped the polls, and insulted Mexicans, Muslims, women, his opponents and the media. He's attracted crowds willing to wait hours to see his unscripted addresses, but will they come to caucus and vote in 2016?
Here is a sampling of what's made Trump so memorable as a candidate:
"When Mexico sends its people, they're not sending their best. They're not sending you. They're not sending you. They're sending people that have lots of problems, and they're bringing those problems with us. They're bringing drugs. They're bringing crime. They're rapists. And some, I assume, are good people." -- June 16, announcement of his candidacy
"Donald J. Trump is calling for a total and complete shutdown of Muslims entering the United States until our country's representatives can figure out what is going on." -- Trump campaign statement, Dec. 7
"I watched when the World Trade Center came tumbling down. And I watched in Jersey City, New Jersey, where thousands and thousands of people were cheering as that building was coming down. Thousands of people were cheering," Trump said Nov. 22, at a rally in Birmingham, Alabama.
Of Megyn Kelly's questioning during the first Republican debate, Trump told CNN, "You could see there was blood coming out of her eyes, blood coming out of her wherever."
The year of the outsider
Traditionally, government experience has been considered a qualifying factor for the presidency. And throughout his term in the White House, President Obama's critics called him too inexperienced to handle the job. Yet in 2015, voters' frustration with Washington boiled over to a point where any political experience is considered a negative rather than a positive, particularly within the GOP.
In a September CBS News poll, just 9 percent of Republican primary voters said they wanted a candidate with experience in politics. As many as 48 percent said they preferred a candidate who has experience in business or the private sector.
Not surprisingly, the bombastic billionaire businessman Donald Trump has resonated with GOP voters while spurning the political elite. Retired pediatric neurosurgeon Ben Carson also gained some traction this year, while former Hewlett Packard CEO Carly Fiorina has had standout moments in the GOP debates.
Yet just about every candidate has tried to cast himself or herself as an outsider, whether or not they have experience in Washington or in state politics. For instance, when the GOP candidates introduced themselves at the start of a September debate, there was hardly any mention of any political experience. Sen. Rand Paul introduced himself as "an eye surgeon from Bowling Green, Kentucky." Sens. Marco Rubio and Ted Cruz similarly left out their official titles. New Jersey Gov. Chris Christie declared, "I am a Republican in New Jersey -- I wake up every morning as an outsider."
On the Democratic side of the race, former Maryland Gov. Martin O'Malley has tried to capitalize on voters' interest in outsider candidates. "If you believe that our country's problems and the threats that we face in this world can only be met with new thinking, new and fresh approaches, then I ask you to join my campaign," he said in the second Democratic debate last month, hosted by CBS News at Drake University in Des Moines, Iowa.
So far, however, Democrats are largely backing the ultimate insider: Hillary Clinton, the former first lady, former senator and former secretary of state. Clinton, for her part, tried to argue on CBS' Face the Nation that if elected, she would also count as an outsider: "I cannot imagine anyone being more of an outsider than the first woman president."
Hillary Clinton's emails
If there were any symbol in 2015 of the low-level Washington shenanigans that irritate voters, it may have been Hillary Clinton's email controversy -- both Clinton's misuse of her personal email account and the GOP-driven scrutiny around it.
In early March, it was revealed that Clinton used a private email address and private server to conduct business as secretary of state -- even though government rules say she should have used her State.gov email account. The revelation spurred months of questions about Clinton's email setup, such as whether any classified information was compromised.
The controversy was exacerbated by the fact that Clinton erased about half of her emails -- those which she claims were personal in nature -- before handing over the contents of her email account to government investigators. When asked in August whether the deleted emails were retrievable or whether her server was wiped clean, an exasperated Clinton retorted, "What with like a cloth or something?"
The email controversy gave congressional Republicans to amplify their investigation into the September 11, 2012 Benghazi attack. In October, Clinton was grilled for 11 hours by members of the House Benghazi Committee. The committee chairman, Rep. Trey Gowdy, R-South Carolina, told Clinton at the hearing that her lengthy testimony was necessary because her personal email use had impeded the committee's investigation. "You had an unusual email arrangement with yourself, which meant the State Department could not produce your emails to us," he said.
Clinton conceded it "wasn't the best choice" to use a personal email account as secretary of state, but that, of course, hasn't stopped Republicans from using it as a way to question Clinton's trustworthiness and judgment. "Hillary Clinton is not telling us the truth ... again," a narrator says in an ad the Republican National Committee ran ahead of a Democratic debate this year, hitting Clinton for the email controversy.
Money doesn't matter
The past few election cycles have seen the emergence of astoundingly well-funded super PACs, but 2015 proved that money doesn't always buy political support.
Donald Trump has dominated the Republican field so far, even though he has relied largely on free media attention to get the word out about his campaign. By contrast, Jeb Bush's campaign and his allies have spent more than $40 million so far -- only to watch the former governor flounder in the single digits in polls. "He hasn't spent $40 million. He's wasted $40 million. There's a big difference," Trump said on Tuesday, announcing that he will finally start investing in TV ads.
Jeanne Cummings, political editor of the Wall Street Journal, pointed out on CBS' Face the Nation that Trump has received financial support -- but from grassroots backers giving unsolicited donations. "Small donors can still have a big influence," she said. "The candidates that can draw small donors can survive because that's real support... So we have been reminded that the money primary does matter and that is in the small donor, not necessarily the big donor."