British Prime Minister Theresa May is facing a hard time convincing her lawmakers why the UK should not pursue a customs union with the European Union. This is one of the main issues in the debate over Brexit.

However, many of the most vociferous anti-EU campaigners were absent from a parliamentary debate on urging the government to negotiate “an effective customs union” with the EU.

As reported by the Reuters news agency, their absence suggests a showdown is shaping up to be little more than a painful reminder for May that she is far from winning the argument on Brexit even in her Conservative Party. Also, she could be defeated in parliament later this year.

It was not clear whether parliament would take what would only be a symbolic vote on April 26, but continued support for the customs union among some Conservative lawmakers is a blow to May.

According to May, Britain should leave the EU customs union in order to independently negotiate trade deals with other countries.

However, the UK government has proposed two alternatives: a customs “partnership” and a streamlined customs arrangement.

“We think that both of the options that we’ve put forward can address the issue of the Irish border and of facilitating a smooth customs arrangement with the European Union,” May’s spokesman said earlier this week.