DUBLIN, Sept 28 (Reuters) - The Irish government is close tolosing its parliamentary majority just as it tries to pushthrough reforms to cut the biggest deficit in the European Unionand fix its banking sector.

With elections usually held up to a month after thedissolution of parliament and a new government sitting for thefirst time up to three weeks later, Dublin is in danger ofbecoming a political vacuum.

With members of parliament returning from their summer breakon Wednesday, the following are scenarios that could trigger thefall of the government.

MORE MPS DEFECT OVER SPENDING CUTSPrime Minister Brian Cowen needs to find at least 3 billioneuros ($4.04 billion) worth of savings in December's budget for2011 but cuts implemented this year, particularly in healthcare, have already begun to see his support in parliament fall.

His majority dropped to four seats after an independent MPwithdrew his support last Friday and looked set to slip to twowhen a member of his Fianna Fail party threatened to turn hisback on the fast-sinking coalition a day later.

The government has said it would not strike any deals to getthose MPs back on side so it simply cannot afford any moredefections ahead of the Dec. 7 budget.

After three austerity budgets in two years, Cowen andFinance Minister Brian Lenihan have little option but to cutservices - a move that will really hurt the electorate. Analystssay such a move will force enough defections next year to tipthe balance in a confidence vote and trigger elections beforethe 2012 due date.

OUTSTANDING BY-ELECTIONS ARE HELD

Cowen is under pressure to hold by-elections to fill threeempty lower chamber seats. He is not obliged to call a vote andhas been putting them off as long as he can but indicated thismonth that they would likely take place early next year.

Unless Cowen's party snap a 28-year trend of governing partyby-election losses, the government's majority would all butdisappear.

While the government can only fall if it loses a confidencevote or vote on a financial bill, failing to win a succession ofordinary votes would severely undermine its authority and hastenits exit.

OPPOSITION TAKES ADVANTAGE OF ABSENT MPS

The main opposition party Fine Gael -- which polls suggestwill lead a large majority with the Labour party after the nextelection -- said on Monday it would pull much of the cover itprovides for government MPs absent from parliamentary votes inan attempt to force a snap election.

Although Fine Gael said it would provide cover for MPsabsent due to illness or abroad on matters of nationalimportance, their new stance has upped the ante.

MPS FORCED TO RESIGN FOR HEALTH REASONS

There are at least three Fianna Fail MPs, including FinanceMinister Brian Lenihan, who are battling serious illness and ifany of them has to resign because of ill health that would cutthe government's majority.

Lenihan said earlier this month that treatment for acancerous growth found at the entrance to his pancreas hadstabilised the disease.

JUNIOR PARTNERS PULL THE PLUG, HURT RELATIONS

As the government's majority slowly slipped from a buffer of15 seats just over two years ago, it has become increasinglyreliant on the six members of junior partners the Greens.

Legislation introduced by the Greens often jars with some inFianna Fail and a clash over a stag hunting law saw one seniorpartner lose his party privileges in June after voting againstthe government. Measures to curb corporate donations, expectedlater this year or early next, could be another pressure point.

The Greens, in danger of losing all of their seats at thenext election, might also look for a reason to pull thegovernment down early next year if it would boost theirpopularity.

COWEN LOSES HIS JOB

Both Fianna Fail and Cowen have seen their popularity sinkas the fiscal and banking crises worsened. The prime minister'ssuitability came under scrutiny this month following a row overa radio interview he conducted the morning after partying latewith colleagues.

Some from Cowen's own party have called for him to step downbut no senior minister seems set yet to mount a challenge.Another gaffe could change that with any successful challenger-- who would become this term's third prime minister -- likelyto have to go to the polls shortly after taking charge.

THE BUDGET FAILS

While most analysts and politicians expect the budget tosneak through on a vote and instead see the government fallingsometime next year, were the measures introduced too unpalatablefor enough backbenchers to take, the government would fall.

(Reporting by Padraic Halpin; editing by CarmelCrimmins/Janet McBride)