The Liberal Democrats are heading for their first coalition showdown with the Tories since last week's elections in the House of Lords when they will mount an attempt to delay the introduction of elected police commissioners.

The move will be the first material test of the Lib Dems' weekend promise to be a "louder voice in government" after last week's disastrous election results.

It comes as Nick Clegg is due to reaffirm his party's support for refugees and asylum seekers. In a speech at the Refugee Council's 60th anniversary celebrations, Clegg is to argue that "Liberals, progressives and true patriots" must reclaim Britishness and not surrender its meaning to "bigots and xenophobes like the BNP".

The deputy prime minister is also to criticise the "constant hum of cheap populism" from politicians of right and left and parts of the media that vilifies refugees and paints asylum seekers as a threat to the country.

Labour have made clear they will back Lib Dem amendments to the police bill in the Lords that seek to delay the first elections for police commissioners due next May by calling for their introduction to be piloted in up to six police force areas first.

The home secretary, Theresa May, slapped down the Lib Dems' home affairs spokesman, Tom Brake, who backed the demand for pilots and argued that stronger "robust checks and balances" were needed to ensure that the commissioners did not interfere in operational police matters.

She said the new police and crime commissioners would be introduced in every police force across England and Wales outside London next May.

The move follows mounting concern from senior police officers, including the Metropolitan Police Commissioner, over the threat to their operational independence posed by elected commissioners.

Two Lib Dem amendments are scheduled for debate and a possible showdown vote during the committee stage of the police bill.

The first, by Baroness Hamwee, calls for pilots to be held in six police force areas each lasting two years before a final decision is taken to roll them out nationally.

The second by Lord Bradshaw is tougher urging that two 3-year pilot schemes be introduced from next May which would take the final decision beyond the next general election. It is possible – but not guaranteed – that the debate will go to a vote.

Labour said they would support any Lib Dem amendment on pilots for police commissioners. The shadow home secretary, Yvette Cooper, said Clegg had called for a "'louder Liberal Democrat voice'. Well it sounds like they are shouting, but she [May] is not listening."

In his Refugee Council speech Clegg is to argue that the Coalition is "unwinding the damage of years of mismanagement" of the asylum system in Britain. He says that they inherited a system in which innocent children were imprisoned bars and thousands of asylum seekers were forced into a forgotten underclass as their applications were lost in limbo for years.

He argues that this was against a backdrop of "a constant hum of cheap populism. Politicians on the right, on the left, and parts of the media vilifying refugees; lumping anyone seeking asylum into the same category; and then painting those people as a threat to the country.

"We must not surrender the meaning of Britishness to the bigots and xenophobes. Liberals, progressives, true patriots must reclaim this ground."

This is language that is unlikely to feature in a David Cameron speech.