2015's Most Memorable Political Moments

— -- Here's a look back at some of the biggest political headlines and moments in 2015:

Hillary Clinton defends her use of private email while in office

On March 10, Clinton held a press conference at the United Nations in which she answered questions about using private email while she was Secretary of State. “I opted for convenience to use my personal email account, which was allowed by the state department, because I thought it would be easier to carry just one device for my work and for my personal e-mails instead of two,” she said.

Obama announces historic Iran nuclear deal

“Today, the United States, together with our allies and partners, has reached a historic understanding with Iran, which, if fully implemented, will prevent it from obtaining a nuclear weapon," the president said April 2.

Donald Trump announces he's running for president

"So ladies and gentlemen, I am officially running for president of the United States, and we are going to make our country great again," Trump said from Trump Tower in New York City.

With his announcement speech on June 16, the business mogul stirred up a bit of controversy over comments he made about Mexican immigrants, but he has remained a frontrunner in the GOP.

Supreme Court legalizes gay marriage

In a historic decision, the Supreme Court ruled 5-4 on June 26 that same-sex couples have the constitutional right to get married. Just a day before, the Supreme Court ruled to uphold Affordable Care Act subsidies.

The U.S. re-establishes diplomatic relations with Cuba

On July 1, Obama announced the re-establishment of U.S.-Cuba diplomatic relations and the re-opening of the embassies in each respective country. Then on Aug. 20, the Cuban flag was raised in Washington DC for the first time in over 50 years.

The stage was crowded at the first GOP debate

A whopping 17 people announced their candidacy for the Republican nomination for president this election cycle. The first Republican debate, with 10 candidates on stage, drew a record of 24 million viewers.

Pope Francis visits the U.S.

In his first visit to the U.S., Pope Francis traveled to D.C. to meet with Obama and give a speech on Capitol Hill.

“I am most grateful for your invitation to address this Joint Session of Congress in ‘the land of the free and the home of the brave,” the pontiff said Sept. 24.

House Speaker John Boehner resigns

The day after Pope Francis wrapped up his visit to D.C., Boehner surprised many by announcing he would be stepping down as House Speaker at the end of October. "This morning I woke up and I said my prayers as I always do and I said, 'today, I'm going to do this,' and that was that," the Ohio congressman said of his Sept. 25 decision.

Bernie Sanders and Hillary Clinton agree at first Democratic debate

During the Oct. 13 Democratic debate, Vermont Sen. Bernie Sanders declared to Clinton "the American people are sick and tired of hearing about your damn emails." Clinton laughed, and said she agreed. “Thank you," she said. "Me too."

Vice President Joe Biden announces his decision to not run for president

The vice president said on Oct. 21 that the window for a realistic White House bid closed before his family was able to work through the grieving process surrounding the May death of his son Beau. Biden's announcement from the Rose Garden that day ended months of speculation about whether he would jump into the 2016 race.

Paul Ryan sworn in as new House Speaker

Following the San Bernardino shootings and the Paris terrorist attacks, Obama addressed the nation from the Oval Office Dec 6. "Over the last few years, however, the terrorist threat has evolved into a new phase," President Obama said. "As we’ve become better at preventing complex, multifaceted attacks like 9/11, terrorists turned to less complicated acts of violence like the mass shootings that are all too common in our society."

Trump stirs up controversy

In a statement released Dec. 7, Trump called for a "total and complete shutdown" of Muslims entering the United States. Trump doubled down on his controversial policy at a rally he held in South Carolina that night.

Sanders apologizes to Clinton for campaign's breach of voter data

In the Dec. 19 Democratic debate hosted by ABC News, Sen. Sanders apologized to Clinton for a campaign staffer accessing and downloading Clinton's private voter data due to a bug in the firewall of the DNC’s software.

"Yes, I apologize. Not only do I apologize, I want to apologize to my supporters. This is not the kind of campaign that we run," Sanders said on stage when asked by ABC's David Muir whether Clinton was owed an apology.