On Monday, Sen. Marco Rubio (R-Fla.) announced his candidacy for president in Miami, becoming the fourth candidate to officially enter the race and the third from the Republican Party. Rubio is the junior U.S. senator from Florida and was first elected to the Senate in 2010. He quickly became a Tea Party darling and delivered the Republican response to President Obama's State of the Union address in 2013. Rubio's campaign is reportedly positioning the 43-year-old senator to be the "youthful face" in his party, which has struggled to win over young voters as well as minorities. In his announcement speech, he directly challenged Hillary Clinton -- and, indirectly, his likely Republican opponent Jeb Bush -- by saying "yesterday is over" and that other leaders are "stuck in the 20th century."

So, where does Rubio stand on millennial issues?

1.Education

Marco Rubio laid out his higher education plans in a February 2014 speech at Miami-Dade College, sponsored by the National Journal. The speech echoed bipartisan themes: the economic importance of higher education and the growing cost of college.

"So among the most pressing challenges before us today is to transition to a new model of delivering higher education that equips Americans for the better paying careers of this new economy.

"One of the central problems of our outdated higher education system is that it has become increasingly unaffordable for those who stand to benefit the most. Tuition rates have skyrocketed at a rate far exceeding the rise in inflation. Even when the Great Recession took hold 5 years ago and Americans had less to spend, the rise in tuition only continued to accelerate. Between 2006 and 2012, the cost of college increased by 16.5%."

The senator also highlighted his own large student loan debt from college and law school and his use of Pell Grants. The speech can be watched below and read in full here.

Rubio's plan appears market-driven. It includes establishing "an independent accrediting agency to assess free courses offered over the Internet and elsewhere as transferrable credits," according to the Huffington Post. His goal is to have internships, online classes, and other experiences -- if approved by the accrediting agency -- eligible for federal aid.

The plan also included "student investment plans," which would consist of funding from the private sector, not the government. Approved private investors "would pay your $10,000 tuition in return for a percentage of your income for a set period of time after graduation – let’s say, for example, 4 percent a year for 10 years," Rubio stated in his speech. He also would like students to be able to analyze how much they'll make in a field compared to how much they'll pay in loans.

With respect to the millions of Americans stifled by student loan debt, Rubio strikes a more conservative tone. In 2014, he was one of 37 senators who helped block Sen. Elizabeth Warren's (D-Mass.) Bank on Students Emergency Loan Refinancing Act. When asked by National Journal about allowing student borrowers to declare bankruptcy, Rubio gave a vague response:

"I think we have to examine that, but we should be careful about it too, only because I think that can make it harder for others to access loans in the future if the lending becomes uncollectible. I think a better approach is to allow people to pay back based on how much money they make, because that will incentivize people to continue paying the loans rather than to go into default, which will damage their credit."

His response was just was wishy-washy on the topic of forgiving student loan debt:

"We have that now in some targeted and specialized fields, and that's a concept that should be perhaps expanded in some unique circumstances. I would be wary about saying you're going to forgive massive numbers of student loans because I think that would make future student-loan lending much more difficult."

2. Marijuana

Sen. Rubio is against both legalizing and decriminalizing marijuana.

He has said that he believes there is no "responsible way to recreationally use marijuana." When asked in an ABC/Yahoo interview about states that have legalized recreational use of the substance, Rubio stated that the federal law, which treats marijuana as an illegal substance, should be enforced.

"Marijuana is illegal under federal law," the Florida senator stated. "That should be enforced. I believe that adding yet another mind-altering substance to something that's legal is not good for the country."

When asked if he has ever tried smoking marijuana, Rubio was more evasive.

"Here's the problem with that question in American politics," Rubio said in that same ABC/Yahoo interview. "If you say that you did and suddenly there are people out there saying 'Well, it's not a big deal. Look at all these successful people who did it.' I don't want my kids to smoke marijuana. And I don't want other people's kids to smoke marijuana. I don't believe there's a responsible way to recreationally use marijuana."

Video of the interview is below:

Rubio still hasn't provided a solid an answer about his stance on medical marijuana, saying in January of 2014 that he'd like to hear more on the "science of it."

"You hear compelling stories of people who say the use of medicinal marijuana provides relief for the thing they are suffering," Rubio said in an interview back in 2014, when a medical marijuana initiative was on the Florida state ballot. "So I'd like to learn more about that aspect of it, the science of it."

"I have qualms about that proposal, I really do, but I probably need to learn more about it," Rubio continued. "The broader issue of whether we should be legalizing it is something I'm pretty firm about. I don’t think legalizing marijuana or even decriminalizing it is the right decision for our country."

3. Immigration

Immigration has been one of Marco Rubio's key -- albeit fraught -- issues during his time in the Senate. In short, he was forced to move his position to the right.

Initially, Rubio positioned himself as an immigration reformer, seeking to help Republicans reach out to Latino voters who overwhelmingly supported Barack Obama in the 2008 and 2012 elections. Rubio, who believes in "a path to citizenship," helped write a 2013 bipartisan immigration bill, which passed the then-Democratic-controlled Senate, but died in the Republican-controlled House of Representatives. He has also stated that DREAMers -- undocumented immigrants who came to the U.S. as children -- are "real people," who he has met with.

But times have changed. After his Senate immigration bill failed in the House, mostly due to its unpopularity with the conservative base of the Republican Party, Rubio distanced himself from his bill as well as DREAMers. At the Conservative Political Action Conference (CPAC) earlier this year, Rubio wholly abandoned the bill, admitting he was wrong about his original immigration position and that he has "learned" from his mistake. (For more, ThinkProgress created a timeline of his immigration stance.)

"You have 10 or 12 million people in this country, many of whom have lived here for longer than a decade, have not otherwise violated our law other than immigration laws, I get all that," Rubio stated at CPAC. "But what I've learned is you can't even have a conversation about that until people believe and know, not just believe but it's proven to them that future illegal immigration will be controlled."

4. Environment

Studies have shown that climate change will greatly impact Rubio's home state of Florida. Nevertheless, Rubio says that he does not believe humans are causing climate change.

"Our climate is always changing," Rubio stated in a May 2014 with ABC's "This Week." "And what they have chosen to do is take a handful of decades of research and say that this is now evidence of a longer-term trend that's directly and almost solely attributable to man-made activities."

"I do not believe that human activity is causing these dramatic changes to our climate the way these scientists are portraying it," he later stated in that same interview. "I do not believe that the laws that they propose we pass will do anything about it, except it will destroy our economy."

BuzzFeed dug up an old video of Rubio, which reveals that this is a starkly different stance from the one he took in the Florida statehouse. Back in 2007, Rubio stated that Florida should try to be the "Silicon Valley" of clean energy and understood that carbon emission caps were inevitable. Watch the video from Climate Desk below:

5. LGBT Rights

Sen. Rubio supports "traditional marriage" and reiterated that point during a 2014 speech at the Catholic University of America in Washington, D.C.

"There is a growing intolerance on this issue, intolerance of those who continue to support traditional marriage," Rubio said, according to Politico. "Even before this speech is over, I’ll be attacked as a hater or bigot. Or someone who’s anti-gay. This intolerance in the name of tolerance is hypocrisy. Supporting the definition of marriage as one man and one woman is not anti-gay. It is pro-traditional marriage."

In the same speech, Rubio also condemned discrimination of the LGBT community and stated that gay marriage should be up to the states.

"Those who support same-sex marriage have a right to lobby their state legislatures to change state laws," Rubio stated. "But Americans who support keeping the traditional definition of marriage also have a right to work to keep the traditional definition of marriage in our laws without seeing that overturned by a judge."

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