On the morning of the Martin Luther King Jr. holiday, Sen. Kamala Harris announces her candidacy.

"Let's get you up to touch with some breaking news this morning. Senator Kamala Harris going big with this," Hallie Jackson said on Morning Joe.

I am running for president of the United States. And I'm very excited about it. This is a moment in time where i feel a sense of responsibility to stand up and fight for the best of who we are.

"It is official. That is going down just as other 2020 hopefuls are making appearances to honor the legacy of Dr. Martin Luther King Jr.," Jackson said.

"NBC political editor Mark Murray joining us now. Our panel is back with us too. She's making the announcement out on GMA, breaking the news. She's got an event coming up later this week or early next. Where does this go?"

"This goes in the field keeps getting bigger and bigger. This presents a very diverse field. You now have your third female Democratic senator who's in, joining Kirsten Gillibrand as well as Elizabeth Warren. You have the Latino former mayor of San Antonio, Julian Castro. We're waiting to see what Joe Biden does as well as Beto O'Rourke. The central question is going to be who's best equipped to be able to beat Donald Trump and beat him on the stage 2020? To me, that's going to be the year-long search for an answer for that question for Democratic voters.

"Is it someone like Kamala Harris, a former prosecutor, state attorney general, U.S. senator who's been in office for two years? Is it someone like Beto O'Rourke, someone who's an experienced person, like Joe Biden? Is it a fighter like Elizabeth Warren? We're going to have debates, TV ads, contests to decide that. There are some advantages that Harris brings to the field. Not only does she have that diversity where she is African-American, an Indian-American, which could help her with African-American voters who are key. Also California has an outsize role in the Democratic nomination process, given its early move and the delegate allocation map that goes on. And so she has some advantages. We're going to see how all these people play out and what the final size of this field is," Murray said.