BOULDER, Colo. (AP) — President Barack Obama has a message for college students who don't like Republican Mitt Romney: "Don't boo. Vote."

The Democratic candidate on Sunday took his campaign message to the University of Colorado in Boulder, a liberal enclave in a state seen as a critical battleground this November. With rolled shirt sleeves, Obama cast the election as a chance for young Americans to ensure that his health care law is enacted and women and gay rights are protected.

He told the cheering crowd that Americans are greater together than they are on their own and that, in his words, "we've come too far to turn back now."

When the crowd booed at the mention of Romney's name, Obama urged them to vote and then referred them to a voter registration website.