There is a Nexus lane both ways at Lewiston, although into the US is not very useful as it's only open for two hours each day.
Into Canada: 7 days a week 8am - 8pm
Into USA: M-F 7am - 9am
Coming back into Canada is great because you use the truck lane over the bridge (Truck/bus/RV/Nexus). So you fly by the one lane of parked cars.