There was a time when you could see Donald Trump as a joke who would disappear from the political scene as soon as responsible Republicans got serious about who they really wanted to see as president.

The joke is over. After leading in virtually every poll for the last four months, Trump is the Republican front-runner, and what he says has to be taken seriously. It’s way past time for responsible Republican leaders to disassociate the party of Lincoln from the inflammatory rhetoric and religious bigotry that he has unleashed. Otherwise, we can assume that he really does represent Republicans in the 21st century.

Donald Trump has shown bigotry not seen in a political leader since the days of legal segregation. The Associated Press

Trump’s transgressions started the day that he announced for president, when he accused immigrants of being murderers and rapists, and promised to deport 11 million people by force. In the week since the Paris attacks, he has sunk to new lows.

First he proposed shutting down Muslim places of worship as a way to fight terror.

Then he agreed with the notion that all Muslims should be forced to register in a national database. He later said that he was only responding to a reporter’s question, but when asked if there should be such a database, his response was: “I would certainly implement that. Absolutely.”

We have not seen such open bigotry by a political leader since the days of legal segregation. You would expect that Republican office holders would want to disassociate themselves from this dangerous demagogue. But instead their criticism has been aimed at President Obama.

Normally, that is what they should be doing. We have a two-party system, and it is important for the opposition party to make its case in a robust debate.

But when the leader of one of those parties is openly calling for oppression based on race and religion, it’s time for the members of his party who find that unacceptable to make themselves known.

Republicans can’t dismiss Trump as a loony on the fringe anymore. He has sustained too much support for too long to be treated as entertainment. If responsible Republicans can’t steer their party away from Trump now, they will have to accept all that comes with the association. And that is not a joke.