Generally speaking, in the LDS church, you're not really supposed to pick your congregation. Instead, every geographic area is divided up into "wards" and so you'd just attend whatever ward in which you reside. So technically the White House is inside the geographic area of some ward in the DC area, and so somewhere nearby there would be a chapel where people in that ward attend. If Romney were to win, that's where he'd attend. When traveling, you just locate a nearby chapel and attend there. 
 
Sunday meetings are open to the public, so the Secret Service people along with any of the President's constituents would be welcome.