WASHINGTON — President Donald Trump announced Tuesday that his administration is taking action to cap insulin costs for seniors with diabetes - a move that comes as polls show Trump lagging behind former vice president Joe Biden, the presumptive Democratic nominee, with that group of voters.The administration has brokered an agreement between insulin manufacturers and some Medicare prescription drug plans that would lower insulin costs for some seniors by capping co-pays at $35 for a monthly supply for insulin - a figure that administration officials said would lead to about a two-thirds drop in out-of-pocket costs and would encourage seniors to be able to continue taking insulin.“We have reached a breakthrough agreement to dramatically slash the out-of-pocket costs of insulin,” Trump said from the Rose Garden during the late-afternoon event.