It's a long ride from LV to LP straight through Death Valley. Best is to rent a car.
If you fly into Burbank instead, can take Metrolink train to Lancaster, switch to ESTA bus. But ESTA bus schedule is very limited -- one bus a couple days a week (and not on weekends).
Also can fly into Reno and take ESTA bus, again, limited schedule.
Having a rental car will also help get to the Portal from LP. There is no public transportation from LP to the portal. You can hitch, pay a local shuttle driver, or drive yourself. Or you can walk the 10 miles. That's about it.