For two years there have been stories that the Rudd Government would call an early election. Most of these stories have been easy to dismiss, first because they seemed to originate from the Opposition rather than the Government, and second because they ignored that the Government was constitutionally prevented from calling an early election.

Now as the pointy end of the electoral cycle approaches, Prime Minister Kevin Rudd faces a decision on whether to call a normal election where half of the Senate faces the people, or to roll the dice and call a double dissolution.

In my view the Prime Minister is most likely to call a double dissolution election for August or early September. There are issues to weigh up on timing and possible Senate results, but the compelling reason why the Prime Minister will call a double dissolution election is that it is the only way he can bring an immediate end to the blocking power in the Senate of the Coalition and Family First's Steve Fielding.

The timing problems of a double dissolution go back to the 1890s and the origins of the Constitution. The double dissolution power contained in Section 57 of the Constitution is not just an excuse for an early election. Section 57 is one of the key provisions that made federation possible.

The 1890s constitutional conventions agreed on creating a variable-term House of Representatives where states were represented according to their population, and a fixed-term Senate with staggered terms in which the states would have equal representation. But having given the small states so much power in the Senate, the larger states only agreed to federation if a government responsible to the popularly elected House of Representatives was given a mechanism to overcome an obstructionist Senate. The mechanism produced was the double dissolution, a provision with no parallel elsewhere in the world.

This piece of 19th-century history about states' rights may sound irrelevant today, but it is critical to understanding the timing issues the Rudd Government faces with this year's election.

First is the problem of the Senate's fixed term. Writs for a half-Senate election cannot be issued until one year before the end of the Senate term. The retiring half-Senate was elected in 2004 and its term expires on June 30, 2011. So the Government cannot issue a writ until July 1, 2010, meaning the first possible date for a House and half-Senate election is August 7, 2010.

Of the 36 state senate seats facing election, 19 are held by the Coalition. The 2010 election will undo the 2004 election when the Howard government gained a Senate majority. Whether the Rudd Government calls a half-Senate election or a double dissolution, undoing the 2004 result will cost the Coalition seats and deliver the balance of power to the Greens.

The problem for the Rudd Government in a half-Senate election is that the 36 new Senators will not take their seats until July 1, 2011. Calling a normal election would mean no change to the Coalition's grip on the Senate until a year into the Rudd Government's second term. I find it unlikely that the Rudd Government would be prepared to fritter away a third of its second term in office when it has the option to clear the Senate with a double dissolution.

Double dissolutions were designed for deadlocks, not as an alternative to a normal election. For that reason a double dissolution of the Parliament cannot take place in the last six months of a term. Again there are timing issues in this. Parliamentary terms begin on the date of the first parliamentary sitting, not on the date of the last election. The current Parliament first sat on February 11, 2008, so it expires on February 10, 2011 and a dissolution of both houses can take place as late as August 10 this year.

The Government already has its ETS scheme and changes to the private health insurance rebate as double dissolution triggers. The Prime Minister can visit the Governor-General at any time before August 10 and use these bills as the trigger for a double dissolution. It is unlikely Prime Minister would do this before the end of May at the earliest as to hold a double dissolution election before July 1 would cut a year off the Government's second term.

This is because double dissolutions reset Senate terms. After a double dissolution, all Senate terms are backdated to July 1 before polling day. If the Government called a double dissolution election for a date before July 1 this year, Senate terms would be backdated to July 1, 2009, meaning another half-Senate election would have to be held in the first half of 2012. A double dissolution before July 1 would effectively deliver the Government only a two-year second term.

After July 1, the Government would get close to its full three-year term, the next election due in the first half of 2013. Given the constitutional and electoral act restrictions, and the holding of a normal five-week campaign, the Government can hold a double dissolution election between July 3 and September 18 2010 without losing a year from its second term. If the Government wants to go beyond the normal five-week campaign, the election date can be pushed out as late as October 16, as long as the dissolution occurs by August 10.

The key window for the election is the seven weekends between August 7 and September 18, a period when the Government can choose whether it wants to hold a normal House and half-Senate election, or to hold a double dissolution.

A double dissolution elects 12 Senators per state instead of six, reducing the quota for election from 14.3 per cent to 7.7 per cent. That makes it easier for minor parties to win election, and gives the Greens a chance of winning two seats in some states. The corralling of preferences to elect micro political parties, as occurred with the election of Family First's Steve Fielding in 2004, would be easier with the lower double dissolution quota. However, assuming the Rudd Government is re-elected, this lower quota is more likely to cause the Coalition to lose seats to micro-parties rather than Labor.

There are some electoral disadvantages for Labor with a double dissolution. At a half-Senate election, a party wins three of the six seats in each state with 42.9 per cent of the vote. Winning six out of 12 seats at a double dissolution requires a higher 46.2 per cent. That's why Labor would do slightly better out of a half-Senate election than a double dissolution. Under either option, Labor plus the Greens have a majority, but Labor would do better compared to the Greens at a half-Senate election.

Either way, the Greens finish with the balance of power. Labor might do better out of a half-Senate election, but it would not remove the Senate's Coalition block, nor give access to a joint sitting to pass the blocked legislation.

One final advantage of a double dissolution is the allocation of six and three-year terms to the new Senators. If Labor wins, they are more likely to finish with three six-year Senate terms in most states, and only two or three three-year Senators. The Coalition is more likely to win more of the 3-year Senate terms, giving the Labor Party a slight advantage at the 2013 election.

The Government has its constitutional triggers for a double dissolution, but it may not want these to be the key political justification. The ETS legislation has become more controversial since the collapse of the UN climate change talks last December, and the private health insurance changes are a breech of a 2007 election promise.

If the Government calls a double dissolution, it needs to turn the focus from its constitutional justification to its political justification, which is breaking the ability of Senator Fielding and the Coalition to block and delay legislation. Already the Government is ramping up its rhetoric about Senate obstructionism.

Some final timing issues are redistributions and state election timetables. Draft electoral boundaries are due to be released for Victoria at the end of July. Issue of writs for a double dissolution will cancel the redistribution process.

Writs for a Victorian election will be issued on November 2, effectively setting October as the last realistic month for a Federal election. The New South Wales Electoral Commission wants to start rolling out its new automatic enrolment procedures, and an election held too late this year will create confusion with the new New South Wales procedures.

The Rudd Government can hold off until April 16, 2011 if it wants, but that means fighting on new Victorian electoral boundaries, creating confusion with the New South Wales enrolment laws and holding an election at the same time as the doomed New South Wales Labor Government. That won't happen.

Political logic points to a double dissolution after July 1, most probably in August or early September. The Government has constitutional justification for a double dissolution, and is building up its political justification based on Senate obstructionism. It has the timing opportunity of holding a double dissolution without losing a year from its term and polls that give it a chance to remake the Senate. All the logic points to the Rudd government taking its chance.

Antony Green is the ABC's election analyst.

More on possible election dates and Senate results at Antony Green's blog.