The Liberal Democrats are "seriously in discussions" with Plaid Cymru to agree a pact at the next general election, Lib Dem leader Jo Swinson has said.
A poll is expected before Christmas because Boris Johnson's government has no majority in the House of Commons.
The Liberal Democrats won the recent Brecon and Radnorshire by-election after Plaid agreed not to stand.
Ms Swinson told BBC Wales discussions would "take their course", before details on specific seats are revealed.
Opposition MPs have twice rejected the government's attempts to trigger a general election, insisting they want to ensure the UK does not leave the EU without a deal at the end of October before they vote for an election.
Conservative ministers have accused them of running away from facing voters.
Following the Lib Dems by-election victory, there has been talk of a establishing a wider pact for the general election in an attempt to unify the vote for a further referendum on EU membership.
Asked how that was progressing, Ms Swinson told BBC Sunday Politics Wales: "There are constructive discussions underway…but those discussions will take their course and announcements will be made as that happens."
Speaking ahead of her party's autumn conference, beginning in Bournemouth this weekend, the Lib Dem leader declined to name constituencies where such a pact could operate, but she acknowledged the Plaid-Lib Dem marginal seat of Ceredigion would be exempt.
Plaid Cymru Ben Lake currently holds the west Wales seat with a majority of 104. Ms Swinson said she expected a "lively fight" in Ceredigion.
A spokesman for Plaid said: "Plaid Cymru is Wales' leading pro-Remain party…..We will continue to work across party lines to deliver sensible, grown-up solutions to end this Brexit chaos."
But Ms Swinson insisted the Liberal Democrats would compete against pro-Remain Labour MPs even in marginal seats,
"Labour is not a Remain party….despite the efforts of some excellent Labour MPs who have tried their best….as Liberal Democrats we need to make sure people have a choice to vote for a party that genuinely wants to stop Brexit."
On the other side of the Brexit debate, Downing Street has poured cold water on Nigel Farage's proposal of an electoral pact between his Brexit Party and the Conservatives.
A senior Conservative source said Mr Farage was "not a fit and proper person" and "should never be allowed anywhere near government".
Mr Farage accused the Tories of "petty, tribal, party politics".
The full interview can be seen on BBC Sunday Politics Wales, broadcast at 10:00 BST on 15 September and will then be available on BBC iPlayer.