Liberal Democrat leadership hopeful Ed Davey has ruled out forming any kind of pact with other Remain parties and independent MPs who want the UK to stay in the EU, saying they must join his party and try to influence it from within.

The former cabinet minister said the Lib Dems had proved their strength in the recent local and European elections, and that any formal pact would compromise the party and reduce the clarity of its messages. Davey is running against the party’s current deputy leader, Jo Swinson, who is regarded as the favourite, in the race to succeed Sir Vince Cable who is standing down in July.

Speculation over whether a new Lib Dem leader would be open to joining some form of Remain alliance at the next election has grown following recent elections, and the decision last week by six of the 11 MPs who had formed Change UK, to split from the fledgling party and sit as independents.

Supporters of a pact argue that only by clubbing together can the Remain side prevent its vote splitting, and unite those who want a second referendum under the same banner.

But Davey told the Observer that a pact of any sort would not be on offer if he became leader. He said: “I admire these MPs for putting political principles before political careers. But the Liberal Democrats smashed it in the local and European elections, and as Labour tears itself apart over Brexit, we are the only viable party of Remain.

“A pact doesn’t cut it. The Liberal Democrats should be very confident about our strength and our values. Anyone on our side who suggests a pact would be selling the Liberal Democrats short. A pact would simply blunt our clear anti-Brexit, pro-environment message. These MPs need to prove their liberal values and join the Liberal Democrats. If they disagree with us on anything, try to change us from within.

We have boots on the ground, donors and, above all, passionate supporters Ed Davey, Lib Dem leadership contender

“They now have serious thinking to do. A chill wind blows for independent MPs, and they have left two parties in a few months. We have boots on the ground, donors and, above all, passionate supporters.”

Davey says he would want to spend what he calls a £15bn “no-Brexit dividend” on deprived areas “left behind by the Tories”.

He added: “I would welcome the campaigning skills of the likes of Sarah Wollaston [the former Conservative and Change UK MP] and Chuka Umunna [the former Labour and Change UK MP] to join our crusade to show how stopping Brexit will end austerity and help the most vulnerable.”

Swinson, who is also against the idea of a formal pact but has been more open to the possibility of working with other parties, said: “I want to lead the Liberal Democrats to build a liberal movement. That requries us to reach beyond our current base and bring on board the millions of people who want to stop Brexit, believe immigration is a good thing, and want to tackle the climate emergency. The European and local election results have proven the Liberal Democrats will be at the heart of that movement and anyone who shares our values should join us.”