Republican presidential candidate Mitt Romney has picked Wisconsin Congressman Paul Ryan to be his running mate, according to a Republican with knowledge of the development.

The official spoke to The Associated Press on condition of anonymity because this person was not authorised to disclose the decision. Romney's completion of the Republican ticket comes as he tries to repair an image damaged by negative Democratic advertising and shift the trajectory of a campaign that's seen him lose ground to President Barack Obama.

The vice presidential selection will dominate headlines, and Romney's team has been relentlessly teasing the announcement for weeks.

Ryan, 42, is viewed by some in the Republican Party as a bridge between the buttoned-up party establishment and riled-up conservative activists that have never warmed to Romney.