Here's a roundup of news trending across the nation and world today.

What to know now:

1. Republican debate: The remaining Republican candidates for president will meet in Houston Thursday night for the last debate before Super Tuesday voting next week. Donald Trump, Ted Cruz, Marco Rubio, Ben Carson and John Kasich are set to appear onstage in the CNN-hosted debate.

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2. ISIS threat: Facebook founder Mark Zuckerberg and Twitter CEO Jack Dorsey were the targets of threats in a video purportedly released by a group connected to the Islamic State terror organization. The "sons of the Caliphate army" released the video which shows photos of Zuckerberg and Dorsey being riddled with bullets. The group said it objects to efforts to remove posts and accounts from the two social media platforms.

3. Bombshell returns: Mitt Romney, Republican nominee for president in 2012, is calling out Donald Trump over his failure to release his tax returns for public scrutiny. According to Romney, he believes Trump has not yet released the returns because  "there's something there. Either he's not anywhere near as wealthy as he says he is, or he hasn't been paying the kind of taxes we would expect him to pay."

4.  Supreme Court nominee: According  to a source involved in the process, President Barack Obama is considering a moderate Republican as a potential nominee to fill the Supreme Court seat left vacant when Justice Antonin Scalia died earlier this month. Nevada Gov. Brian Sandoval is said to be among those being considered to fill the spot on the Supreme Court. Sandoval is a Republican and was once a federal judge.

5. Deadly weather: Four people died Wednesday as a line of storms responsible for three deaths Tuesday continued up through the Mid-Atlantic states. Two men and a boy were killed in Waverly, Va.,  after the mobile home they were in was hit by a tornado. A man in Appomattox County, Va., was killed when his home was destroyed by the storm. More than 2,500 flights have been canceled in the Northeast Thursday due to the weather.

And one more

Michael Skakel,  the nephew of Ethel Kennedy, was in court Wednesday, trying to clear his name in the 40-year-old murder of a 15-year-old Connecticut girl. Skakel's attorney claimed Wednesday that Skakel's brother was the  one who killed  Martha Moxley in 1975, and that his  client was innocent and should not be sent back to jail. Skakel, convicted in 2002 of the murder, served 11 years for the crime,  but was released on bail when a judge said his then-attorney made major mistakes in the trial.

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