Sen. Marco Rubio, R-Fla., announced Wednesday he is running for re-election to his Florida Senate seat. Alan Diaz/AP hide caption
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Sen. Marco Rubio, R-Fla., has until June 24, his state's filing deadline, to make up his mind. Paul Sancya/AP hide caption
Donald Trump speaks at Fountain Park during a campaign rally on March 19, 2016 in Fountain Hills, Ariz. Ralph Freso/Getty Images hide caption
Sen. Marco Rubio, R-Fla., pauses while addressing the American Conservative Union's Conservative Political Action Conference in National Harbor, Md., earlier this month. Cliff Owen/AP hide caption
Bernie Sanders and Hillary Clinton stand together Wednesday before the start of the Democratic presidential debate at Miami Dade College in Miami. Wilfredo Lee/AP hide caption
Republican presidential candidates, Sen. Marco Rubio, R-Fla., left, Donald Trump, Sen. Ted Cruz, R-Texas, and Ohio Gov. John Kasich, right, stand together during the signing of the National Anthem, before the start of the Republican presidential debate Thursday in Miami. Wilfredo Lee/AP hide caption
Donald Trump reacts during a campaign rally in Fayetteville, N.C., Wednesday. Gerry Broome/AP hide caption
Democratic presidential candidate Sen. Bernie Sanders scored an upset win in Michigan on Tuesday. He's seen here speaking last week in Illinois, which holds its primary next Tuesday. Seth Perlman/AP hide caption
Bernie Sanders addresses a rally at Macomb Community College in Warren, Mich. Sanders needs a good showing in Michigan on Tuesday to start to turn his campaign around. Scott Olson/Getty Images hide caption
Republican presidential candidate, Sen. Marco Rubio, R-Fla., speaks in Toa Baja, Puerto Rico, on Saturday. Paul Sancya/AP hide caption
Marco Rubio and Donald Trump participate in the Republican presidential debate in Detroit. Geoff Robins/AFP/Getty Images hide caption
Donald Trump gestures Thursday as Sen. Marco Rubio, R-Fla., listens to Trump's response during a Republican presidential primary debate in Detroit. Paul Sancya/AP hide caption
Donald Trump addresses a crowd in South Carolina. Trump has the clearest path to the GOP nomination, but Republicans opposed to him aren't giving up the fight. Spencer Platt/Getty Images hide caption
Donald Trump addresses Marco Rubio in one of their many heated exchanges during a debate last week in Houston. Gary Coronado/AP hide caption