Trump had refused to rule out a third-party run during the Aug. 6 Republican debate.
NEW YORK -- Donald Trump will sign a pledge to back the Republican presidential nominee and not to run as an independent should he not win the party's nomination, multiple outlets have reported.
18:04 With his presidency off course, Trump confronts the lessons of the health-care debacle21
07:05 SEE IT: Ads praising Republicans for replacing Obamacare aired hours after Trump-backed bill flopped22
17:32 #TrumpTruck memes take over the Internet after photo of Trump in a semi-truck emerges15
00:51 Trump Campaign Was Subject to 'Incidental' Surveillance, Says Nunes28