Sen. Marco Rubio address the Heritage Action Presidential Candidate Forum on Sept. 18, 2015 in Greenville, S.C. (Sean Rayford/Getty Images)

Marco Rubio is a U.S. senator. And he just can’t stand it anymore.

“I don’t know that ‘hate’ is the right word,” Rubio said in an interview. “I’m frustrated.”

This year, as Rubio runs for president, he has cast the Senate — the very place that cemented him as a national politician — as a place he’s given up on, after less than one term. It’s too slow. Too rule-bound. So Rubio, 44, has decided not to run for his seat again. It’s the White House or bust.

“That’s why I’m missing votes. Because I am leaving the Senate. I am not running for reelection,” Rubio said in the last Republican debate, after Donald Trump had mocked him for his unusual number of absences during Senate votes.

Five years ago, Rubio arrived with a potential that thrilled Republicans. He was young, ambitious, charismatic, fluent in English and Spanish, and beloved by the establishment and the tea party.

Sen. Marco Rubio, who's running for president in 2016, is known for his stances on immigration and tax reform. Here's the Florida Republican's take on Obamacare, the Islamic State and more, in his own words. (Julie Percha/The Washington Post)

But Rubio had arrived at one of the least ambitious moments in Senate history and saw many of his ideas fizzle. Democrats killed his debt-cutting plans. Republicans killed his immigration reform. The two parties actually came together to kill his AGREE Act, a small-bore, hands-across-the-aisle bill that Rubio had designed just to get a win on something.

Now, he’s done. “He hates it,” a longtime friend from Florida said, speaking anonymously to say what Rubio would not.

Which makes for an odd campaign message.

Rubio must convince voters that his decision to leave the Senate — giving up the power he already has — is actually a mark of character, a sign that he is too dedicated to public service to stay.

Rubio is not a quitter, the argument goes.

In fact, that’s precisely why he’s quitting this place.

“He wouldn’t be doing what he’s doing now if he were a quitter,” said Norman Braman, a Florida auto dealer and one of Rubio’s longtime donors.

Watch Marco Rubio during a West Miami City Commission meeting in 1999. It was his first job as a public official. (The Washington Post)

Last week was — by Rubio’s standards — a very busy week in the Senate.

On Tuesday, he cast a vote, his first in 26 days. He gave a floor speech, his first in 41 days. In the speech, he asked the Senate to pass a bill that would give Department of Veterans Affairs leaders more power to remove poor-performing employees. But it didn’t happen. To pass something so suddenly, Senate rules required that every single senator agree. A Democrat, Sen. Richard Blumenthal (Conn.), objected that Rubio’s bill did not allow workers due process.

“Unfortunately, we will not be able to move forward on this today, it appears,” Rubio said, as his gambit failed. With that, he was done for the week, missing the next three votes.

Impatience had been a hallmark of Rubio’s career, for good and ill. Even in his first elected office, as a young city commissioner in West Miami, Rubio had been exasperated by how slowly his colleagues worked.

In this case, his decision to give up his Senate seat in 2016 was, in some ways, forced upon him.

The state’s election filing deadlines might have allowed him to test the presidential waters and then — if he failed — drop out and run for reelection as a backup plan. But even that Plan B was far from a sure thing. Rubio would be facing an expensive campaign in a competitive state, having already made clear that the Senate race was his second choice.

On the campaign trail, Rubio comes under attack from rivals who say he’s become an absentee federal employee. Former Florida governor Jeb Bush, in a less-than-subtle knock on his former homestate ally, has said senators who miss work should have their pay docked.

“It’s just, kind of, like, dude, you know, either drop out or do something,” Bush’s son, Jeb Bush Jr., told New York University College Republicans earlier this month, in comments first reported by Politico Florida. The junior Bush, a Floridian, cast himself as an aggrieved constituent. “We’re paying you to do something, it ain’t run for president.”

Rubio waves off the criticism, saying his rejection of the Senate is all about finding a better way to serve all Americans.

“I’m not missing votes because I’m on vacation,” he told CNN on Sunday. “I’m running for president so that the votes they take in the Senate are actually meaningful again.”

Asked about his absences recently by Matt Lauer on NBC’s “Today” Show, Rubio said: “My ambitions aren’t for me. My ambitions are for the country, and Florida.” If he is elected to the White House, he added, “we can begin to fix some of these issues that I’ve been so frustrated we’ve been unable to address during my time in the Senate.”

In an interview with The Post, Rubio was asked to go further. What if he didn’t see an opportunity for himself in the presidential race? Would he run for reelection, if the Senate was all he had?

“I don’t know,” Rubio said.

To understand how Rubio got to this point, The Post interviewed him, his friends and his staffers, and examined his nearly five-year-old Senate record — votes, speeches, legislation and committee hearings.

It is clear that Rubio’s frustration started early.

“Do we just stand around and do nothing?” Rubio said on the Senate floor in November 2011. “Or do we actually begin to act?”

That was 10 months in.

Rubio had arrived in January, after a stunning defeat of Charlie Crist, Florida’s sitting governor. Rubio had previously held the job of Florida House speaker, which had put him in the middle of one legislative fight after another.

He was used to action.

But in Washington at that point, action was not an option. The House had been taken by Republicans. Democrats still had the Senate. And those Democrats — who’d previously passed huge bills on Wall Street reform, health care and financial stimulus — saw little upside to crafting new, complicated bills. That might force vulnerable Democrats to take votes that could be used against them. And, if they did, the House would probably kill the bills anyway.

So the Senate fell into a bitter somnolence, interrupted by moments of panic.

It mainly took action when Congress’s gridlock threatened to shut down the government or cause a national default. In Rubio’s first two years, the Senate held just 486 recorded votes. That was the fewest in 20 years, according to the publication Vital Statistics on Congress.

Rubio had proposed an ambitious freshman agenda: cutting spending, rolling back EPA rules in Florida, even a clever legislative trolling effort to cut the money for signs that said this-and-such project was bankrolled by President Obama’s stimulus bill.

All failed. Rubio’s only successful bill that year was one to name September as National Spinal Cord Injury Awareness Month.

So, 10 months in, Rubio and Sen. Christopher A. Coons (D-Del.) unveiled an idea for a fail-proof bill. They assembled 10 job-growth ideas that were effectively pre-chewed: They were known to be bipartisan and uncontroversial. The AGREE Act.

“It went nowhere,” said one former Rubio staffer.

Rubio’s staff blamed its failure on the Republicans and Democrats on the Finance Committee, which had jurisdiction over the bill but ignored it.

In December of that year, a Florida reporter asked Rubio about the highlights of his first year in office.

“I can’t think of a single real high point,” Rubio said.

In his second year, Rubio had better numbers. He got four bills through the Senate. But all were symbolic resolutions, including such controversial items as “A resolution congratulating the Miami Heat for winning the National Basketball Association Championship.”

In his third year, however, it appeared that Rubio’s moment had finally come.

He joined a bipartisan “Gang of Eight” that negotiated a massive immigration-reform effort. He argued with talk-radio hosts, trying to sell conservatives on a plan that would give illegal immigrants a path toward citizenship.

Then, when the bill came to the Senate floor, Rubio gave the speech of his career. He used his own parents’ story: They emigrated from Cuba and worked as a bartender and a maid while Rubio was growing up. He talked about the American dream — and how immigrants treasure it and nurture it.

“Here in America, those who once had no hope will give their kids the chance at a life they always wanted for themselves. Here in America” — he tapped the lectern — “generations of unfulfilled dreams will finally come to pass. And that’s why I support this reform,” Rubio said.

The bill passed, giving Rubio the kind of victory that most freshman senators could only dream of.

For a moment, it also seemed to validate his strategy of working within the slow-moving Senate’s limitations. Two freshmen with presidential ambitions — Republican Sens. Rand Paul (Ky.) and Ted Cruz (Tex.) — had rejected that go-along strategy and instead used marathon speeches and other tactics to stop the system in its tracks.

But soon, Rubio’s moment was over.

The House Republicans didn’t move on immigration. Rubio didn’t push them. Then, Cruz seized the spotlight, as he started a parliamentary game of chicken that ended with a government shutdown.

After that, friends and staffers started to see something different in Rubio. There was a new level of frustration and a new sense that he was looking beyond the place. In his Senate floor speeches, Rubio talked often about his family’s humble backstory. In late 2013 and 2014, he told the Senate six different times that his father had been a bartender.

His colleagues already knew, of course. But Rubio was aiming at people who didn’t know — people on the other side of a TV camera.

In the past couple of years, some of Rubio’s other policy ideas have actually become law.

His Girls Count Act, which prods developing countries to register girls’ births and give them greater property rights, became law on his second try. His idea to allow the VA administrator greater freedom to fire incompetent leaders also became law, tucked into a larger bill.

But it’s also been clear that Rubio’s ambitions were aimed elsewhere.

He began missing votes. He skipped 10 percent of them in 2014 — making him one of the most absent senators, with the 88th-best attendance record, according to statistics kept by GovTrack.us. He began missing committee work, even on the subject he most identified with. In 2014, Rubio missed 34 of 68 committee hearings and meetings in the Foreign Relations committee, according to his office’s tally.

When Rubio tells the story of his career, the moment that he gave up on the Senate does not come until later.

“For two years, we just tried to slow-dance and wait for the 2012 election,” in the hopes of winning a Republican majority, Rubio said. “And then, when that didn’t work out, we spent two years trying to position ourselves for ’14.”

Then Republicans did win a majority that year. But, Rubio said, he was still told to be patient. The majority was still too small to overcome Democratic filibusters or override presidential vetoes.

“Now it’s [2015], and the argument is, ‘We’ve gotta wait to elect the president,’ ” he said.

That meant the same frustrations, under a new boss. And that was it.