Jo Swinson has announced she will run to become the next leader of the Liberal Democrats.
The party’s deputy leader told BBC Question Time the country was “crying out” for a movement “to challenge the forces of nationalism and populism”.
She faces competition from former Energy Secretary Sir Ed Davey, who has declared his intention to stand.
Current leader Sir Vince Cable will step down on 23 July and nominations to replace him close on 7 June.
The pro-EU Lib Dems came second in the UK’s European elections last week, taking 20.3% of the vote.
Ms Swinson, MP for East Dunbartonshire and a former equalities minister, was thought likely to stand when former leader Tim Farron quit in 2017 but ruled herself out and went for the deputy position instead.
Asked if she would be running, she told BBC Question Time: “Yes, I am going to stand to be leader of the Liberal Democrats.
“I will set out my plans tomorrow [Friday] but I happen to think that this country is crying out for a liberal movement that will challenge the forces of nationalism and populism and the Liberal Democrats need to be at the heart of that movement and I’m the person to lead it.”
It comes as a poll of 1,763 adults by YouGov for the Times, on which party they would back if a general election was held now, was topped by the Lib Dems on 24%, ahead of the Brexit Party on 22%.
The Conservatives and Labour were tied on 19% and the Greens on 8%. The poll, conducted on 28 and 29 May, also found that nearly 70% of voters believed Brexit was the most important issue facing the country.
Candidates for the Lib Dem leadership must be an MP supported by at least 200 members spread across at least 20 different local parties.
They must also have the backing of 10% of the parliamentary party – currently, the party has 11 MPs.
Lib Dem members will then have the final say on who becomes leader.