Conservative MSP John Lamont is to resign his Holyrood seat to fight for one at Westminster in June's election.
Mr Lamont has been selected as the Tory candidate to take on the SNP's Calum Kerr in Berwickshire, Roxburgh and Selkirk, Scotland's most marginal seat.
He announced he will quit as the MSP for Ettrick, Roxburgh and Berwickshire on 4 May, triggering a by-election.
Holyrood's presiding officer will set a date for that vote in due course, but it could also potentially be on 8 June.
The UK goes to the polls that day in a snap general election.
Mr Lamont has been an MSP since 2007, and currently serves as the chief whip and parliamentary business manager for the Scottish Conservatives.
He has contested the Berwickshire, Roxburgh and Selkirk seat on three occasions already, having lost to Mr Kerr by 328 votes in 2015, with both men beating the sitting MP, former Scottish Secretary Michael Moore, into third place after a recount.
The MSP said the election was "one of the most important general elections Scotland has ever had".
He said: "In the Borders, it will be straight fight between myself and the SNP. I have decided to stand down as an MSP to allow me to focus all of my energy on this important contest and to ensure that the electorate have a clear choice between the two contenders.
"I've taken this decision because I believe that I am best placed to help increase the number of Scottish Conservative and Unionist MPs at Westminster so we can stop the SNP bandwagon in its tracks. With this decision, no one can doubt my commitment to representing the Borders to the best of my ability."
Mr Kerr said he welcomed the opportunity to "make a renewed case for representing the region I'm proud to call home".
He said: "Since I was elected, I have fought first and foremost for the issues that matter most to my constituents.
"On the front bench at Westminster and in our local communities, I have stood up for the interests of the rural economy, taken forward the pressing issue of digital connectivity and worked with people of all parties, and none, on issues that matter to the Borders.
"I cannot overstate how much of an honour it has been to represent the people of Berwickshire, Roxburgh and Selkirk over the past two years."
The Lib Dems meanwhile have selected Catriona Bhatia, currently deputy leader of Scottish Borders Council, as their candidate for the seat, which was held by the party for more than 50 years.
She said: "Having grown up in the area, it is an honour to be selected to be the local candidate.
"The Borders has a strong Liberal tradition, and I have already been speaking to many voters who are keen to come back to support the Lib Dems this time as they are fed up with the politics of division which is being promoted by the Tories and the SNP.
Three more MSP have so far confirmed they will stand in the Westminster election, all Conservative colleagues of Mr Lamont - Douglas Ross, Ross Thomson and Miles Briggs.
None have yet indicated they will quit and as all three men are list MSPs none would, in any case, trigger a by-election.