The Democratic Unionist Party became the largest party in Northern Ireland following the assembly election in 2003.
DUP leader Ian Paisley: An MP since 1970
The party, which opposed the Good Friday Agreement, overtook the Ulster Unionist Party by securing 30 seats, a 33% increase on its 1998 election result.
Its dominance in the assembly increased with the defection of senior Ulster Unionist MP Jeffrey Donaldson and two other UUP assembly members following the election, bringing its membership in the assembly to 33.
The party has seven MPs after taking three seats from the UUP in North Belfast, East Londonderry and Strangford in the 2001 election.
It draws its support from the Protestant community and is fiercely opposed to any moves towards involving the Republic of Ireland in Northern Ireland affairs.
The DUP has refused to enter a power-sharing executive with the largest nationalist party, Sinn Fein, as it views it as being "inextricably linked" to the IRA.
A deal to restore devolution failed last December after the party demanded photographic proof of IRA decommissioning. Sinn Fein opposed the demand for photographs, saying republicans would not take part in a process aimed at "humiliating" them.
The party is engaged in a long-running bitter battle with rival unionist party the Ulster Unionists, whom it claims betrayed unionism by entering government with Sinn Fein before full decommissioning of IRA weapons had been achieved.
Party leader Ian Paisley, who is also head of the Free Presbyterian Church, was elected to Westminster in 1970.
At that time he represented the Protestant Unionist Party which was formed in the late 1960s.
In 1971 it was re-created as the Democratic Unionist Party.
Then the party announced it would be "right wing in the sense of being strong on the constitution" but "to the left on social policy".
The DUP is defending the Westminster seats held by Mr Paisley in North Antrim, Peter Robinson in East Belfast, Nigel Dodds in North Belfast, Jeffrey Donaldson in Lagan Valley, Gregory Campbell in East Londonderry, and Iris Robinson in Strangford.
It is suggested that the DUP may enter a pre-election pact with the UUP in order to maximise the anti-Sinn Fein vote and avoid a split in the unionist vote. Two constituencies thought to be among those considered for a pact are South Belfast and Fermanagh-South Tyrone.