The move was a surprise, as Republican candidate Trump has yet to choose his running mate, but it has focused attention on the enhanced role of vice presidents nowadays. Once selected purely to win support from a particular region, they can now play significant roles in presidential administrations.
Trump will formally introduce his running mate to the party at the Republican National Convention, to be held in Milwaukee in mid-July.
The debate between the vice-presidential candidates will be watched closely, as much of the public assumes the president may need replacing.
Given the two-term limit on presidents, the winning vice president will be a front runner for nomination to run for the presidency in 2028.
Trump aims to replace Mitch McConnell, who steps down as Republican Senate leader, with someone able to work well with his vice president.
