* Nigeria's ruling party loses majority in lower house
* Move comes during difficult week for president
By Camillus Eboh
ABUJA, Dec 19 (Reuters) - Thirty seven Nigerian lawmakers have defected to Nigeria's main opposition coalition, giving it a slim majority in the lower house of parliament, in a further blow to President Goodluck Jonathan's 2015 re-election bid.
The move follows a defection by five powerful governors last month to the opposition All Progressive Congress (APC) and a scathing denunciation of Jonathan by former President Olusegun Obasanjo that has emboldened both dissenters within his party and the opposition.
"We the undersigned members of the House of Representatives elected under the platform of the Peoples Democratic Party (PDP), wish to inform you that we have joined the APC," they said a letter dated Wednesday.
With the move, the PDP now has 171 members in the lower house, while the APC has 172, although the defecting parliamentarians may yet lose their seats as a result of the switch, leaving the PDP in control.
The PDP has been in power since shortly after the end of military rule in 1998, but it has been riven by internal squabbles centred on Jonathan's assumed intention to run for another term in office in polls expected in 16 months.
Many northerners say Jonathan's running again would violate an unwritten PDP rule that power should rotate between the largely Muslim north and mostly Christian south every two terms.
He has also come under fire for his record on tackling the Islamist insurgency in the northeast and fighting corruption.
Despite the defections, most analysts expect Jonathan will win the vote if he chooses to run, albeit with a weaker mandate.
But the tighter the race, the more money is likely to be spent fighting it at a time when Nigeria's fiscal position traditionally worsens as leaders seek to secure votes.
Reuters has seen a bid for a court order preventing the lawmakers from being stripped of their seats as a result of the defection. Twenty two ruling party senators had their name on the plea, suggesting they too planned to defect, although they were not immediately available to comment.
Obasanjo, 76, was one of Jonathan's sponsors, but in the letter leaked last week he chided the 56-year-old and wrote that it would be "fatally, morally flawed" for Jonathan to seek re-election in 2015.
He likened the corruption on his watch to that under military dictator General Sani Abacha, whose rule was marred by looting of funds from Africa's biggest oil producer.
The presidency rejected the comments as baseless.
Jonathan, a southern Christian, was vice president and came to power when President Umaru Yar'Adua, a northern Muslim, died in May 2010, three years into his first term.
The perception, strongly denied by the presidency, that corruption has worsened under his rule has also been damaging.
Diplomats, politicians and newspaper columnists were disappointed when Jonathan granted a pardon to ally Diepreye Alamieyeseigha, a former state governor from Jonathan's oil-producing home state of Bayelsa, who was convicted of stealing millions of dollars of public money.
The Senate is investigating billions of dollars that the state oil company has failed to remit to the government, although the central bank said the figure was $12 billion on Wednesday, less than the $50 billion initially estimated. (Writing by Tim Cocks; Editing by Joe Brock and Mike Collett-White)