It's official: The Holyrood election starts now. There won't even be time for MSPs to shake hands and, however grudgingly, wish each other a Happy New Year before they find themselves pitted against each other in the fight for Holyrood seats. For today isn't just the day they return to parliament after their festive break. It is the first day of the "regulated period" leading up to polling on May 5, when the Electoral Commission's strict rules on spending and campaigning come into force.

The battle for Holyrood will dominate the next 17 weeks.

Nicola Sturgeon set the ball rolling at the weekend with an article in The Herald's sister paper, the Sunday Herald, which signalled a presidential-style SNP campaign designed to capitalise on her unprecedented personal approval ratings.

Kezia Dugdale will today focus on housing when she makes her first big election speech in Edinburgh, an event Ms Sturgeon hopes will be overshadowed by her own New Year debate at Holyrood. The First Minister will make education a top priority, just as Scots Tory leader Ruth Davidson did when she set out a series of schools policies yesterday. The Lib Dems' Willie Rennie, meanwhile, has been talking about a "green and liberal" future during a visit to a factory in Fife.

So much for Holyrood's "main" parties. What of the Scottish Greens who, based on the most recent polling, look set to overtake the Lib Dems to become the parliament's fourth biggest party?

They are furious at being classified as a "smaller party" in the BBC's draft guidelines for its election coverage, a move that would restrict them to significantly less airtime than their opponents.

The BBC's rules, designed to ensure fair and impartial reporting, kick in on March 23, by the way.

For the Scottish Greens, much is at stake. The pro-independence party has polled strongly and grown in size since the 2014 referendum. Averaging out last year's polling figures, it could expect to win nine seats under Holyrood's semi-proportional system, its biggest ever tally.

Yet the party's confidence is fragile. The Greens know they could be squeezed by the new left-wing RISE (Respect, Independence, Socialism and Environmentalism) party, which includes members of the Radical Independence Campaign and is allied to the Scottish Socialist Party, and by the SNP's determination to win not just constituencies but regional seats too. When Ms Sturgeon felt moved to write "those who support the SNP have not been brainwashed," her message was aimed at those who might be inclined to lend their list vote to a more authentically radical party.

The Greens have made their case against being classed as a "smaller party" in a persuasive briefing for the BBC Trust, which is consulting on the guidelines. It would "fly in the face of clear evidence of political support and credibility," Greens leader Patrick Harvie complains.

In its draft guidelines, the BBC accepts audiences will expect it to cover the Greens "more substantively" than other smaller parties, though that could still limit them to one appearance a week on programmes usually packed with politics.

The consultation on the BBC's guidelines runs until January 15. After that, the corporation has a difficult decision to make.