Swinson will tell the Liberal Democrats' conference that she can defeat "dictator" Boris Johnson and "Brexit by nature" Jeremy Corbyn.
Chuka Umunna says he will remain in the Liberal Democrats for the rest of his life despite defecting from both the Labour party and Change UK this year.
Lib Dem MP Layla Moran said she would welcome former Change UK MPs into her party with open arms.
An extraordinary new YouGov poll put the Liberal Democrats in first place, with the Brexit Party second and Labour and the Conservatives trailing.
The anti-Brexit Lib Dems recorded their best ever European election results while the Green Party also surged.
Lib Dem leader Sir Vince Cable urged Change UK to put aside "petty tribalism" and form an electoral alliance with other anti-Brexit parties.