But world number one Tiger Woods, a 14-time major golf champion chasing the all-time record of 18 majors won by Jack Nicklaus, lurked only four strokes off the pace in a share of 13th with the third round looming Saturday afternoon.
Woods seeks a record-tying fourth US Open crown but has never won any title when over par after 36 holes and has never won a major crown when he was not leading after 54 holes, making his impending 18 holes even more crucial.
The storm-hit showdown at Merion, thrown off schedule after play was halted for more than four hours on Thursday, continued in perfect conditions Saturday with 73 players making the cut at eight-over par 148.
Mickelson, a five-time US Open runner-up still chasing the elusive trophy, and fellow American Horschel, who missed the cut at the 2006 US Open in his only prior major start, set the overnight pace at one-under 139 for 36 holes. England’s Luke Donald and Justin Rose were in the clubhouse just one stroke back on 140.