Sen. Rand Paul (R-Ky.) announced Wednesday that he is suspending his presidential campaign, bringing an end to a bid that began with aspirations of expanding the libertarian base that his father, Ron Paul, built into a powerful national coalition.

"It's been an incredible honor to run a principled campaign for the White House," Paul said in a statement. "Today, I will end where I began, ready and willing to fight for the cause of Liberty."

The low-key senator, as apt to quote a philosopher as to quote Pink Floyd, struggled in a year dominated by hard-line outsiders such as Sen. Ted Cruz (Tex.) and businessman Donald Trump. In a call with reporters, his campaign strategists bemoaned that the mogul had sucked the "oxygen" out of the race, and admitted hat his non-interventionist views on foreign policy were not embraced by Republicans as terrorism and unrest raged abroad.

Paul, a first-time candidate for national office, also found it difficult to persuade the supporters of his father — a former congressman from Texas who sought the Republican presidential nomination in 2008 and 2012 — to back him with enthusiasm. Although popular among libertarians, purists in that wing of the conservative movement questioned whether Paul was too mainstream.

“His decision to move to the middle and support a number of moderate Republicans in the 2014 elections, that sent us a real signal about the application of his ideology,” said Drew Ivers, Ron Paul’s 2012 Iowa chairman, who did not endorse Rand Paul.

“He muddled on his message, tried to get the left, and I told him it would backfire… I just saw someone trying to do too much and be too many things to too many people. His name is Paul, I told him. You can’t defy gravity."

Paul, a 53-year-old ophthalmologist, was elected to the Senate in 2010 as part of the GOP's tea party wave. He will return to his work in the chamber and to his Senate reelection campaign, where he remains a top target of Democrats as they try to retake the majority this year.

According to Paul's campaign aides, the candidate decided to drop out after a disappointing finish in Monday's Iowa caucuses, where his father had placed a strong third in 2012.

Over the past day, Paul began to inform his key aides and donors that he would leave the race, then returned to Washington, D.C. to resume his work in the Senate. In his statement, Paul expressed confidence that his campaign, while unsuccessful, offered Republicans an alternative by focusing on issues he thinks are crucial to attracting minorities and young people to the party.

"Across the country thousands upon thousands of young people flocked to our message of limited government, privacy, criminal justice reform and a reasonable foreign policy," he said. "Brushfires of Liberty were ignited, and those will carry on, as will I. Although, today I will suspend my campaign for President, the fight is far from over, I will continue to carry the torch for Liberty in the United States Senate and I look forward to earning the privilege to represent the people of Kentucky for another term."

Longtime associates of Ron Paul framed Rand Paul’s departure from the race as a stumble rather than a crippling defeat in the younger Paul’s career — and for their movement. Paul ally Trygve Olson said that libertarians and other conservatives probably would consider Paul in a future presidential cycle that was less crowded and driven by a celebrity front-runner.

“Rand Paul remains the best advocate those conservative have in the United States Senate,” Olson said. “Even if he didn’t have the kind of success he wanted, he’s an important voice on a whole set of issues. Assuming he’s reelected, he will continue to be that voice and be the person putting them at the center of the radar.”

Olson said Ron Paul’s star turn in the 2008 race was driven as much by the national political environment as it was by the candidate’s performance. “When Ron ran then, he was able to tap into the angst that was out there about the war and foreign policy,” he said.

Paul's method of campaigning echoed his father's. He favored small gatherings where he could make lengthy speeches and preferred orations that cited Montesquieu. While he was accessible to the media, he did not relish banter with reporters or the glad-handing demands of the campaign. Some candidates used every spare minute to meet potential voters; reporters who ran into Paul at an airport usually caught him reading a book.

But his ambitions were grand. Two years ago, he became the first Republican to assemble a network in all 50 states as a precursor to a presidential run — a sign that he was looking to build a coalition that wasn't as ad hoc as his father's.

Magazine covers proclaimed Paul to be one of the party’s brightest and most interesting voices — a hemp-wearing, curly-haired maverick, who could, in his words, spread the Republican message to "voters with tattoos and without tattoos."

He began courting Wall Street titans and Silicon Valley entrepreneurs who had donated to the presidential campaigns of George W. Bush and Mitt Romney, attending elite conclaves in Utah and elsewhere. His pitch combined his antagonism toward the National Security Agency’s surveillance programs with what he called “crunchy conservatism,” a prioritization of drug-sentencing reform, the environment and civil liberties.

In 2013, as the Republican Party was casting about for new leadership, Paul filibustered the nomination of the new FBI director and only quit the floor ten and a half hours later, after the White House put out a letter clarifying whether American citizens could be targeted by drone attacks. He got mixed reviews for a speech to black students at Howard University, but then, hunkering down, he reached out to black political leaders in Kentucky and began talking more about criminal justice reform. Republican Party chapters tapped Paul to open offices in poverty-stricken parts of Louisville, Chicago, and Detroit.

“I’ve been to Ferguson, and I've been to the south side of Chicago,” he told a largely white audience of Republicans in Baltimore, after the death of Freddie Gray in police custody sparked two days of rioting. "Until we've walked in someone else's shoes, we shouldn't say we can't understand the anger of people."

Few Republicans talked like that. In early 2014, Paul was consistently leading — or close to leading — national polls. A CNN/ORC International survey in March of that year found that 16 percent of Republicans and independents who lean Republican were likely to support the senator, putting him at the front of the Republican field, and giving him unusually strong support from black voters. At home in Kentucky, Paul loaned strategist-turned-nephew-in-law Jesse Benton to the re-election campaign of now-Senate Majority Leader Mitch McConnell, signaling that he had bridged a gap between the "liberty movement" and the Republican elite. Some pundits, like The Atlantic's Peter Beinart, suggested that Paul was really the frontrunner to win a changing party's nomination and challenge presumed Democratic nominee Hillary Clinton on traditionally liberal turf.

“Right now I'm the only one that beats Hillary Clinton in certain purple states,” Paul told Fox News host Megyn Kelly shortly before his launch. “I'm the only one that also scores above all the other Republicans in whether or not I can beat her.”

Paul’s May 2015 announcement tour started in Lousiville, and swept into the first four primary states, finding sizable crowds. His campaign, led by Republican strategist Chip Englander, had a plan to finish at least second or third in all of the early states, then take advantage of the long-stirring Paul organizations in the Midwest and Mountain West to collect delegates.

That never happened. Donald Trump, who entered the primary one month after Paul, charged ahead in polls and captured the “outsider” energy Paul had claimed for years. Weak poll numbers scared off possible megadonors who had donated to Ron Paul, like the venture capitalist Peter Thiel. In August, right before the first Republican presidential debate, Benton was indicted over a 2012 scheme to flip an Iowa state senator's endorsement to Ron Paul. (Benton was later acquitted.)

Another, more strategic challenge came from a man Paul had endorsed in his bid for a Senate seat in Texas: Ted Cruz. Talking openly about winning the “liberty lane” of the Republican primary, Cruz lobbied supporters of Ron Paul to switch over and back a better-funded campaign.

As he struggled, Paul barely concealed his resentment for Cruz's wrangling. Paul secured a standalone vote on defunding Planned Parenthood; Cruz vacuumed up the attention by calling to cut funds in the must-pass spending bill. Paul won a New Hampshire straw poll of libertarian-leaning Republicans; Cruz claimed victory with his own vote count. Paul told audiences that Cruz had been “nowhere to be found” on a vote to audit the Federal Reserve; voters did not seem to care. This past Sunday, when the elder Paul joined his son for a “revolution”-themed rally on the University of Iowa campus, most of the press seats were empty; only C-Span bothered to film a teleprompter-delivered closing message.

Paul's early exit calmed Republicans who worried that his race for the White House was risking control of his Senate seat, which is on the 2016 ballot. Paul had worked hard to protect his day job, paying $250,000 from his campaign account to fund a March presidential caucus, which he now will not contest. At the “revolution” rally, early Paul endorser Rep. Thomas Massie (R-Ky.) said he was likely to pay for a poll of the state, expecting it to show that Paul could campaign through the next primaries without any risk to his Senate seat.

In a Wednesday radio interview, Cruz praised Paul and reminded listeners that he had always been an ally of the liberty movement. Speaking to reporters in New Hampshire, Sen. Marco Rubio (R-Fla.), a staunch critic of Paul’s foreign policy positions, commended his Senate colleague for running “a good race” and wished him luck in his reelection bid.

“Rand is someone I disagree with on a lot of issues, but as I said the other night at the debate ... he believes strongly in what he stands for, and I respect that,” said Rubio. “He’s a true believer on issues of limited government and the liberty issues and I respect it for him for it."

Asked if he planned to ask Paul for his endorsement, Rubio replied: “Sure. I mean, I’d love to have Rand’s support.”

In a conference call with reporters Wednesday morning, Paul strategist Doug Stafford reiterated that the senator would make no endorsements -- not even of Cruz. It did not matter especially that Paul had endorsed the Texan in his upstart 2012 Senate primary.

"Rand endorsed many people for Senate," he said.

Sean Sullivan contributed to this post, which has been updated.