It's a bold political move by Rosselló. As Puerto Rico's governor, he's overseeing the island's recovery efforts from Hurricane Maria, and the federal government — led by President Donald Trump — is playing an important role in that process. Trump has lashed out over criticism of the federal government's response, and Rosselló's expected decision to back Democratic Sen. Bill Nelson and Democratic gubernatorial nominee Andrew Gillum has the potential to complicate that relationship even more. And a new wave voters from Puerto Rico, displaced by the storm, could have a huge impact on the outcome of both races.Rosselló will first hold an event with Nelson at 10:30 a.m. in Orlando, where Nelson's campaign is teasing a "major announcement." The campaign has not said what that announcement will be and refused to comment directly — but a source with direct knowledge of the event's plans say an endorsement is expected. Separately, a second source with knowledge of Rosselló's intentions confirmed that an endorsement is planned.Later in the day, Rosselló will appear at an event with Gillum. The Gillum campaign is also promising a "major announcement," but would not say if an endorsement is expected.Rosselló is a Democrat, but generally walks a fine line when it comes to his relationships with politicians of all stripes. He has been very cautious to avoid direct criticism of Trump in the wake of the devastation of Hurricane Maria. Rosselló has also had a very good working relationship with Florida Gov. Rick Scott, a Republican, who is challenging Nelson for Senate. Scott has made several trips to the Puerto Rico, including to honor victims on the one-year anniversary of the storm, and Rosselló has made several trips to his state.
CNN's Amir Vera contributed to this report.