Former business secretary Sir Vince Cable has become the first contender to confirm a bid to replace Tim Farron as Liberal Democrat leader.
The 74-year-old said he wanted to use his "energy, enthusiasm and experience" to lead the party through a period of "chronic uncertainty".
The party's elder statesman said he particularly hoped to attract young people "to our cause" with the Conservatives "in disarray and in retreat" and a "complacent" Labour's economic policy set to be "found out".
Former health minister Norman Lamb and former energy minister Sir Ed Davey are both expected to announce challenges against their former Conservative-Lib Dem coalition colleague.
Former business minister Ms Swinson will instead contest the deputy leadership in a move that reportedly could see her support Sir Vince's bid for the top job.
"With the prospect of another election looming large, we must be ready for the fight," he wrote, as he predicted "big opportunities ahead".
Sir Vince added: "We are now growing again and the political winds are moving in our favour."
"To be a political leader, especially of a progressive liberal party in 2017, and to live as a committed Christian, to hold faithfully to the Bible's teaching has felt impossible," Mr Farron said.
The Lib Dems saw their number of seats rise to 12 during the recent election, including Sir Vince's return to a London seat he first won in 1997.
But the party lost former leader and ex-deputy prime minister Nick Clegg on a night of big gains for Labour among the opposition parties.