The tea-party movement in 2010 carried him into Congress, and his views on issues such as gay marriage and abortion are staunchly conservative. Yet it would be misleading to call him either a populist or a "movement conservative" (as he called himself during that campaign), since as a lawmaker he'd go on to advocate for a compromise on immigration.
Because of his willingness to compromise, perhaps along with his hawkish foreign policy and surveillance, he's often seen as part of the "Republican establishment." That label is misleading, though, since he has occasionally sparred with the establishment over his proposals for the tax code.
Instead, like his party's unsuccessful nominee in 2008 -- Sen. John McCain (R-Ariz.), also known as "The Maverick" -- Rubio has often sought his own way in public life, producing detailed proposals on a range of subjects while making plenty of enemies left, right and center.
By one measure, though, Rubio now has a better chance of winning the GOP presidential nod than all of his rivals combined. Here's a look at where he stands on the issues:
In 2013, Rubio worked to broker a compromise in the Senate that would have allowed immigrants living in the country illegally to become citizens. His conservative opponents, including Cruz, denounced the plan as amnesty. Local Republican leaders say that the bill damaged his standing with conservative voters in Iowa and elsewhere.
These days, Rubio still supports eventual citizenship, but he argues that before Congress moves to grant legal status of any kind to undocumented immigrants, the border with Mexico must be secured.
"The American people don't trust the federal government to enforce our immigration laws, and we will not be able to do anything on immigration until we first prove to the American people that illegal immigration is under control," Rubio said during a debate aired on CNN in December.
Rubio called for hiring 20,000 more Border Patrol agents and requiring employers to electronically verify that their employees are eligible to work in the United States.
Rubio's platform reflects traditional family values. He would support a ban on abortion with exceptions for when the life of the mother is in danger, but not in cases of rape or incest. He opposes gay marriage, saying that "God's rules" trump the Supreme Court's decision on the question.
He has also proposed several policies designed to help families financially -- policies that also have proven controversial with some of his fellow Republicans.
He's proposed awarding families a tax credit of $2,500 per child, refundable up to the payroll-tax liability of the parents' employers.
In practice, that means families who would receive a check from the government at tax time would receive a larger one -- but that check would only be increased by up to 7.65 percent of their annual earnings. Families without a parent working formally for a wage or salary would not benefit.
Rubio also wants to subsidize employers that grant workers paid leave to care for a newborn or a sick loved one. Under his plan, the government would pony up a quarter of a worker's pay while on leave, up to $4,000 per worker. Doing so would reduce the cost of offering paid leave, encouraging more employers to adopt the practice.
"One of the greatest threats to family today is that there are too many Americans who have to choose between being there for their children in times of great need or meeting the basic financial needs of their family," the senator said last year.
Policies such as these have drawn criticism from Rubio's more doctrinaire critics on the right. The editorial page of The Wall Street Journal, for example, recently called the tax credit "a vast new entitlement" and "an expensive political pander."
It's not Rubio's only new idea on taxes. He would also eliminate several tax brackets while reducing rates on ordinary income. His plan would eliminate taxes on income from investments altogether, along with the estate tax.
Taken together, Rubio's tax plan constitutes a large tax break for the wealthiest Americans. By one estimate, the plan would increase incomes for the wealthiest 1 percent of Americans by an average of $224,000 a year.
That said, Rubio's plan would also reduce inequality. The plan provides a substantial grant of cash -- up to $2,000 per person -- to most people who file
a tax return. This personal tax credit, as Rubio calls it, would replace the standard deduction and personal exemption in the current system, which taxpayers can use to pay down their tax bills but which cannot be converted to cash. The credit would be reduced for individual taxpayers with at least $150,000 in annual income, and those with more than $200,000 would not receive anything.
That is an important point of contrast with the plans advanced by other GOP candidates, such as Donald Trump. The real-estate magnate's proposal not only would mainly benefit the rich in dollar terms, but the tax break for the wealthy would be so large that, even calculated as a share of their total incomes, they would benefit proportionally more than the working class, increasing inequality.
Under optimistic assumptions, the plan would reduce federal revenue by no less than $2.4 trillion over a decade. That's money the federal government would have to borrow, unless Rubio also made drastic reductions in the budgets of federal programs. The centrist pundit Josh Barro argued Rubio's proposal would be unrealistically expensive, dubbing it the "Puppies and Rainbows Tax Plan."
As an opponent of President Obama's health reform, Rubio has been vocal -- and effective. While his colleagues in Congress have repeatedly tried and failed to repeal the law, Rubio's work on the issue helped eliminate a crucial subsidy for the insurance industry included in it.
Rubio called the subsidy a "taxpayer bailout" for the industry and said that he and other lawmakers who worked to eliminate it had saved taxpayers $2.5 billion. Unexpectedly losing that government money, however, was also a massive headache for insurance companies and their customers, disrupting coverage for at least 700,000 people.
The episode illustrates a dilemma for Republicans. They are united in their opposition to the law known as Obamacare, but have yet to agree on a plan to replace the law without forcing those it insures to pay more or give up coverage.
One of the most popular provisions of the law bars insurance companies from denying subscribers due to pre-existing medical conditions. Insurers adopted this practice because paying for treatment for their sickest customers was proving to be exorbitantly expensive.
Obamacare solved this problem by requiring everyone to buy insurance, an unpopular rule but one that divided the cost of insuring sick people among a much larger group of customers. Rubio's plan would eliminate that rule and create special insurance pools in which the sickest Americans could enroll, sharing the costs among themselves.
These pools already exist in a number of states. They are generally expensive for the people who enroll in them -- premiums are as much as twice the amount paid by healthy people, writes Larry Levitt of the nonpartisan Kaiser Family Foundation, an expert on medical insurance.
And in order to retain customers at those prices, Levitt adds, the pools usually require subsidies from taxpayers. Rubio's campaign has said that the details of how he would pay for the high-risk pools he has proposed are forthcoming.
Rubio also wants to reform Medicaid by taking the money used for the program and giving it to states, along with more flexibility in how those financial resources are used to provide health care for the poor. In the past, Rubio has said that states' grants would be based in part on how many residents need financial help, but his campaign is still working out the formula.
Social Security and Medicare
Starting in about two decades, Social Security will have exhausted its cash on hand, and the program won't be able to pay beneficiaries in full. Rubio has not published a detailed plan for shoring up Social Security's finances, but he has outlined a few ideas about what he would do.
He does not want to make any changes to the program for current retirees, but he wants to raise the retirement age for current workers, and he'd exempt workers over the age of 65 from the payroll tax.
Rubio also wants to offer wealthier retirees less generous annual increases for the increasing cost of living, using some of that money to give more money to retirees who aren't as well off.
With regard to Medicare, Rubio has proposed converting the program into a system of vouchers. The elderly could apply the vouchers either to conventional Medicare coverage or toward a private insurance policy.
Rubio argues vouchers would give retirees more choice in terms of which doctors they see and which hospitals they visit. For liberal proponents of the current system, on the other hand, replacing it with vouchers would be to end Medicare as we know it. "Conservatives' preferred answer to the challenge of paying for Medicare in the future is to scrap the program," writes Vox's Matthew Yglesias.
Contrary to what has been established science for a long time now, Rubio has said that while the planet is getting warmer, global warming is not a result of human activity. "I do not believe that human activity is causing these dramatic changes to our climate the way these scientists are portraying it," he said in 2014.
He has also argued that policies to mitigate climate change would hamper economic activity. "We're not going to make America a harder place to create jobs in order to pursue policies that will do absolutely nothing, nothing to change our climate," Rubio said during a debate in September.
Depending how they are designed, policies that penalize carbon dioxide emissions could be costly in the short term. Economists, however, argue that those costs must be weighed against the consequences of climate change in the long term. The experts generally believe that each ton of carbon dioxide emitted into the atmosphere will end up costing us the equivalent of at least $37 right now -- which comes out to about 37 cents per gallon of gasoline.
"When I am president, what I will do to defeat ISIL is very simple: whatever it takes," Rubio wrote in November. He went on to lay out his strategy: including airstrikes, supporting a multinational military coalition with U.S. troops, and providing arms to Iraq's semiautonomous Kurdish forces.
There are some broad similarities between the strategy Rubio envisions and the one Obama is currently pursuing. As The Washington Post has reported, the United States is covertly sending arms to the Kurds through the Central Intelligence Agency. Obama has sent Special Operations troops to Syria, and American warplanes have been conducting airstrikes against the Islamic State since 2014.
One important difference between Rubio's and Obama's strategies in Syria is Rubio's pledge to establish a no-fly zone to keep President Bashar al-Assad's planes on the ground. Rubio has even said that he would order U.S. fighters to intercept Russian warplanes, which are now flying sorties in the region, to enforce it. Several of Rubio's rivals have taken a similar position, including Ohio Gov. John Kasich, New Jersey Gov. Chris Christie and the former governor of Florida, Jeb Bush.
Obama is opposed to taking such action. So is Cruz, the senator from Texas. Although Cruz has promised to "carpet-bomb" the Islamic State's territory, he has also said he does not want to establish a no-fly zone.