U.K. opposition Labour Party leader Jeremy Corbyn’s policy of constructive ambiguity on Brexit came under pressure Tuesday as his lawmakers argued that Britain should stay inside the European Union’s customs union.

Their calls came when Brexit Secretary David Davis stood in Parliament on Tuesday afternoon to update the chamber on negotiations, after talks stalled on the question of how to avoid putting up a border between Northern Ireland and Ireland.

Labour lawmaker after Labour lawmaker told him that the simplest solution was to stay in the customs union—which facilitates free trade between EU states by ensuring they all charge the same import duties to countries outside and not impose tariffs on countries within.

“There’s a vast majority in this House and in the country and in the House of Lords in favor of us staying in the customs union,” Chris Bryant told Davis. His colleague Liz Kendall added: “The government only has itself to blame for choosing to rule this option out when it doesn’t have to.”

Labour’s official position is vague. Corbyn and his shadow Chancellor John McDonnell say simply that they want a “jobs first Brexit,” and Brexit spokesman Keir Starmer has said Britain should stay in a customs union and the single market during any transitional period.

As Davis was speaking in Parliament, one of Labour’s Brexit team, Jenny Chapman, told the BBC that the party would keep customs union membership “on the table.”

Starmer stuck close to that line in the House of Commons, accusing Prime Minister Theresa May of being reckless in rejecting the option of customs union membership without saying that Labour backed it.

There’s no reason to think the calls will change Corbyn’s position: He has spent more than two years as leader ignoring his lawmakers’ views on most things. But he has also hinted in recent months that he’d be willing to put his own historic doubts about the EU on one side if it might help him to win power.

Clarity could come at a cost: In June’s election, Labour’s vague position helped it to pick up votes both from those who wanted to leave the EU and those who hoped the party would have a softer Brexit stance than the one May proposed.