The extreme groups on either side threaten democracy, but the alt-right and extreme right-wing groups are especially dangerous. The right side wants to suppress people by taking away rights and protections, including voting rights, which undermines democracy by making voting more difficult and less effective. Others need to be wary of these right wing groups.
There has been a huge uptick in extreme right-wing groups with extreme behaviors, and these people tend to think it is okay to behave this way. After seeing a presidential candidate act in a manner that affirms their beliefs, they feel validated in acting the way they do, which leads to this happening more and more every day.
Part of free speech allows for the existence and freedom of expression of those types of groups. Democracy in part means that they get to spew some of their hate messages. However, when they're sending out watchlists and targeting people solely for race or religion, that does threaten democracy and infringe on the rights of others.
World War II was many years ago. In that time, historians and society have had time to study how World War II happened. For this reason, society now knows how to recognize threats and other problems. People will step in before it gets to that point again. The extremist groups will not gain enough power to threaten democracy.