The day after the final Republican debate before Super Tuesday, presidential frontrunner Donald Trump unveiled the endorsement of New Jersey Gov. Chris Christie. Meanwhile, Marco Rubio is ramping up his attacks on Trump, labeling the billionaire businessman a "con artist." Voters in 12 states head to the polls on Tuesday, and half the delegates needed for the GOP nomination are up for grabs.

Following President Obama’s announcement that he will fulfill his longstanding promise to close the detention center at Guantanamo Bay, Capitol Hill lawmakers have been torn between support and opposition. Gwen Ifill talks to Sen. Cory Gardner (R-Col.) and Sen. Dick Durbin (D-Ill.) for their perspectives on the proposed shutdown and what it could mean for the detainees.

President Obama announced plans Tuesday to shut down the detention center at Guantanamo Bay, asserting that its existence undermines national security. The proposal -- which would send the facility’s remaining 91 detainees to domestic U.S. sites -- would fulfill the president’s 2008 promise to close the prison, but Congressional Republicans have been vocal in their opposition.

The latest on the Boston bombing investigation including the arrest of Dzokhar Tsarnaev’s three friends. Also, President Obama renews his commitment to close Guantanamo Bay. Plus, Obama's second-term agenda is being tested by unexpected events. Joining Gwen: Pete Williams, NBC News; Michael Duffy, TIME; Doyle McManus, Los Angeles Times; and Christi Parson of Tribune.