yes it is quite legal...so says the 1st amend...welcome to the world of professional protest and lobbying.
To my certain knowledge this type of thing has been going on in Washington DC for decades. Private interest groups fund buses, T shirts, protest banners, peoples lunches and even give them a few bucks spening money for a days trip to washington Dc to rally or support or protest something or other.
This is a BIG TIME INDUSTRY, there are people who do nothing but organize these types of protests.
Also this is a close cousin to the "flying voter" so common to those of us in the third world...bussing voters in from outlying jurisdictions to vote yea or nay depending on who pays them.