Nimblewill take over
This is a difficult weed to control. If it has taken over more than half of your yard (include other weeds in the calculation), our recommendation is to use glyphosate to kill the entire lawn and reseed. Early fall is the best time. Trying to fight weeds when they have taken over half of a lawn is usually a waste of time and money, because the grass cannot grow back into the spaces left by the herbicide fast enough to out-compete new weeds.
For info on keeping you lawn competative with weeds and how to do a lawn renovation, go to: http://extension.umd.edu/sites/default/files/_images/programs/hgic/Publications/HG102%20Lawn%20Estab...
For a small amount of nimblewill in Maryland, we can recommend Tenacity (a very expensive and sometimes hard to get herbicide). Contact your local extension office for your state's herbicide recommendation.