Sen. Marco Rubio (R-Fla.) plans to officially put to rest questions about his political future on Monday when he launches his run for president in Miami. There are plenty of questions swirling around his presidential bid -- here are the five biggest right now:

Will immigration hurt him in the primary?

Rubio was part of a bipartisan group of senators that led the push to pass a comprehensive immigration reform bill in 2013. The bill, which passed the Senate but went nowhere in the Republican-controlled House, included a path to citizenship for undocumented immigrants that could be problematic for Rubio in the primary campaign. Many conservative activists remain opposed to such a path; and immigration fires up the GOP base like few other issues. Just look at the recent standoff over homeland security funding, in which House conservatives risked a partial government shutdown to fight President Obama's executive actions on immigration. Rubio has since said that pursuing a piecemeal approach to immigration -- rather than the sweeping measure he pursued in 2013 -- is the best way to go. But politics is often as much about what a candidate said in the past than what they have said lately. The good news for Rubio is that one of main expected Republican opponents, former Florida governor Jeb Bush, stands to face as much if not more scrutiny over his moderate positions on immigration. That could take some of the heat off the Florida senator.

Speaking of Bush...

There's no question that Bush's decision to explore a campaign for president has complicated Rubio's outlook. The two are longtime friends and political allies. Their support networks overlap. Bush is a fundraising juggernaut who could crowd out Rubio and others in the fundraising chase. Rubio is no fundraising pushover -- he has the support of billionaire businessman and philanthropist Norman Braman and he raised about $50 million through his three political committees during his time in the Senate. But up against what's expected to be a packed field of candidates including several with with loyal fundraising bases, Rubio is going to need to exceed expectations when it comes to money. Bush's presence could be a major impediment.

How far will his speech-making ability carry him?

In a video previewing his presidential campaign last week, Rubio showcased what is seen by many Republicans as his chief strength: his public speaking ability. The video stitches together key parts of addresses he has given over the years; the idea is to show that Rubio can move people with his words. As the country gets to know Rubio -- polls show he is still largely undefined -- he could set himself apart with memorable speeches. But presidential campaigns are much about the unscripted moments: mingling with voters at the state fair, dealing with hecklers, addressing questions off the cuff at town halls. Rubio will need to do well in those situations and continue to show talent in making speeches to move beyond his underdog status.

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)

How will he balance his day job?

Rubio is one of four Republican senators running for president or considering it, so he's not the only one who will have to balance his job with a national campaign. Sens. Rand Paul (R-Ky.) and Ted Cruz (R-Tex.) will also have to do so. So will Sen. Lindsey O. Graham (R-S.C.), if he decides to run. Rubio is headed back to Washington on Tuesday, where the Foreign Relations Committee he belongs is expected to vote on a critical Iran bill. He will go from campaign focal point to just one of 100 other lawmakers, many of whom have more seniority. Rubio's Senate attendance record has already come under scrutiny -- he is the Senate's biggest vote-misser according to GovTrack -- and it is expected to receive more attention as he runs for president.

What happens if he doesn't win?

Rubio is up for reelection to the Senate in 2016, but he has said he will not try to do that and run for president. So what happens if he doesn't win the Republican nomination? One possibility is that he might be tapped as a vice presidential candidate. Rubio was vetted by Mitt Romney's campaign as a potential running mate in 2012. If he runs a solid campaign and builds a decent following but falls short, his knack for public speaking could put him on the short list once again. Beyond 2016, Rubio has been mentioned as a potential candidate for governor of Florida in 2018. That job stay would allow him to stay on the national radar. Remember, he's only 43. This doesn't have to be his last national campaign.