The SNP received an "immediate lift" after John Swinney announced his bid to be the party's leader, Keith Brown has said. 

The party's depute leader said there had been a positive reaction on the doorstep to the news that Humza Yousaf was set to be replaced by the veteran MSP. 

He also said the opposition had made a tactical mistake by allowing the SNP's “biggest hitter" to take charge. 

READ MORE: John Swinney on cusp of becoming SNP leader and First Minister

Mr Brown's comments came ahead of nominations closing in the leadership contest. 

Over the weekend, there was speculation that activist Graeme McCormick could mount a bid after he secured the necessary number of nominations from members. 

But late on Sunday he told Bylines Scotland that he would back out of the contest after a “lengthy and fruitful” conversation with Mr Swinney.

Speaking to the BBC’s Good Morning Scotland radio programme, Mr Brown said it is important to respect the SNP’s democratic processes.

He said: “It seems to be that the field is cleaning somewhat and it looks like John Swinney might be the sole nominee.”

Mr Brown described Mr Swinney as the “biggest hitter in the SNP” who would scare unionist parties “to a great extent”.

He continued: “I also understand over the course of the weekend, of course, we’ve people out canvassing and they’ve reported already an increase in positive turnout coming back from those canvassing sessions.

“So there’s been an immediate lift from the discussions which John Swinney has had first of all with Kate Forbes and, it now appears, with Graeme McCormick.”

READ MORE: BBC Scotland changes on way as decisions loom for Swinney

It is expected that Mr Swinney could become First Minister as soon as tomorrow. Mr Yousaf will chair his final cabinet before tendering his resignation to the King.

MSPs will then vote for the new First Minister in the afternoon. 

Mr Swinney would then have to swear three oaths of office at the Court of Session in Edinburgh before officially becoming first minister.

Mr Swinney's move into Bute House comes as polling predicts a torrid time for the SNP.

A survey by Norstat – formerly known as Panelbase - found that while support for independence remains largely unchanged, Labour is set to overtake the SNP at both Westminster and Holyrood.

According to the Sunday Times poll, the SNP would win 29% of the vote, while Labour would take 34%.

Professor Sir John Curtice told the paper that meant Mr Swinney's party would hold just 15 of its 43 seats with Scottish Labour winning 28.

Responding to Mr Brown's interview, Scottish Conservative Chairman Craig Hoy said: “Scots are desperate for the next SNP leader to focus on the issues that matter to them such as cutting NHS waiting times, restoring standards in our schools and growing our economy.

“Instead, John Swinney is going to prioritise governing in the SNP’s interests, rather than the country’s interests.

“Keith Brown has a brass neck to say that his next boss will be building on the successes of Humza Yousaf’s leadership, when his time in office was characterised by scandals, endless u-turns and a failure to focus on Scotland’s real priorities.

“In key seats across Scotland at the General Election, it is only the Scottish Conservatives who can beat the SNP, put a stop to their independence obsession and get the focus onto the issues that really matter.”