TAMPA — Republican officials abruptly announced plans Saturday evening to scrap the first day of their national convention, bowing to a threat posed by Tropical Storm Isaac, churning toward Florida.

‘‘Our first priority is ensuring the safety of delegates, alternates, guests, members of the media attending the Republican National Convention, and citizens of the Tampa Bay area,’’ party chairman Reince Priebus said in an e-mailed announcement that followed private conversations involving presidential candidate Mitt Romney’s campaign, security officials, and others.