Our mammoth election campaign has begun, complete with rookie leaders, special sittings of Parliament and a federal budget. And with politics this fluid, there are no sure things anymore, writes Barrie Cassidy.

And so it begins.

From today the country starts out on a strange and unfamiliar 75-day journey to a double dissolution election.

Strange and unfamiliar because it begins with a special sitting of the Parliament, stops off for a federal budget, before flattening out for an almost unprecedented eight week formal campaign.

And all under the guidance of rookie leaders; none of them - Malcolm Turnbull, Bill Shorten, Barnaby Joyce or Richard Di Natale - have ever led a campaign.

In all of those circumstances, nothing can be assumed; nothing ruled out. The days when the election of Malcolm Turnbull at his first try was likely - based if nothing else on the "fair go" factor - are fast fading. He now needs to regroup and convince, just like every other prime minister before him.

The politics are fluid; the electorate clearly volatile.

Take the special sitting, for example. The Prime Minister first proposed the tactic to put maximum pressure on the senate crossbenchers, and crystallise in the minds of the public the justification for a double dissolution election. Apart from that a special sitting was supposed to place all the attention on the need for the ABCC to control rogue unionists and the so-called "protection racket" sponsored by the Labor Party.

It all seemed so one-sided, at first.

But then the major banks played around with interest rates; Bill Shorten seized the moment and called for a Royal Commission into their performance. Suddenly there was another villain in the mix, with one poll suggesting just 18 per cent of Australians oppose the royal commission.

And then compounding the Government's problems the Panama Papers were splashed across the front pages of newspapers. The concept of corporate cheats further diluted the attacks on the unions alone.

But whatever the theatre of the next few days - the budget will be the deal breaker - the document that either saves the day for the Government or justifies the electorate tipping out a first term Government for the first time since the depression.

And making that work politically just got harder for the Government as well.

All year the Government's economic spruikers have insisted the country has a spending problem, not a revenue problem.

You can't tax your way to prosperity, they said.

But then the influential credit ratings service Moody's unloaded its extraordinary warning: that the Turnbull Government had to both cut spending and raise taxes to successfully deal with the problem; and if it didn't it ran the risk of losing its AAA credit rating.

Scott Morrison was then cornered. He changed his rhetoric. He said that of course there would be revenue measures (read tax increases) in the budget, but they would be applied "to reducing the tax burden in other parts of the economy and wherever possible, to continue to drive down the deficit."

The Finance Minister Mathias Cormann on 7.30 - surely in the knowledge that there will be an increase in the tobacco excise in the budget - answered a straight no to the proposition that there will be tax increases in the budget, so confident is he that the electorate will accept the theory that a tax increase is not a tax increase if at the same time the Government offers a tax cut somewhere else.

This budget - so close to an election - cannot be tricky in any shape or form. Trying to re-interpret what constitutes a tax increase is not a good start. The Government would be well advised to ensure today's rhetoric makes the selling process beyond May 3, easier not harder.

Shorten is back in the game essentially because of two things.

In January he relentlessly went after the Government over the option of increasing the rate of the GST. He forced the Government to stop dead in its tracks. That was an important and confidence boosting victory.

Then he put up his own idea, a risky but fundamental change to eligibility for negative gearing. So far, he has effectively protected that politically vulnerable idea from Government attacks.

The Government failed to prosecute the case for unpopular reform; the Opposition stays in the race with its proposal.

When the genuine heat - and the paid advertising comes on - that might change. But as it stands the election is now a contest.

From today, everything matters, and the politicians know it.

Barrie Cassidy is the presenter of the ABC TV program Insiders.