The Ulster Unionists and the DUP have done it for decades. An unofficial pact between the two parties has led to unionists holding 13, and nationalists only five, of the North's Westminster seats.
Today, the SDLP and Sinn Fein will discuss the possibility of adopting a co-ordinated approach. Sinn Fein says it could result in nationalists taking six additional seats.
If more nationalists than unionists were elected to the British parliament, it would have a huge psychological effect on unionists.
Delegations from the SDLP and Sinn Fein will meet at Stormont this afternoon to discuss an alliance but the chance of success is minimal. The meeting is Sinn Fein's idea. The SDLP says it is attending only "out of courtesy".
It is accusing Sinn Fein of engaging in a "cynical ploy" to capture media attention and fool the electorate into viewing it as the champion of pan-nationalist unity and the SDLP as the spoiler.
SDLP chairman Mr Alex Attwood says: "Sinn Fein is as interested in an election pact as Ian Paisley is in becoming a Catholic."
Sinn Fein insists it is genuine. The party chairman, Mr Mitchel McLaughlin, says the vast majority of nationalist voters want a deal to secure maximum nationalist representation. Current demographics allow for a healthy contest between Sinn Fein and the SDLP in five constituencies - West Belfast, Foyle, Newry and Armagh, South Down and Mid Ulster - without allowing the seat to go to a unionist.
Seven constituencies - North Antrim, South Antrim, East Antrim, East Belfast, Lagan Valley, Strangford and North Down - are solidly unionist. However, the remaining six constituencies are winnable because of nationalist majorities or a fragmented unionist vote, according to Sinn Fein.
These are West Tyrone, North Belfast, Fermanagh and South Tyrone, South Belfast, Upper Bann and East Derry. Sinn Fein hasn't publicly stated the constituencies in which it would like the SDLP to stand aside but it has pointed out that in the 1998 Assembly elections it was ahead in the first three. The problem is that these are the bigger nationalist constituencies. The constituencies in which the SDLP is ahead - South Belfast, Upper Bann, and East Derry - are the smaller ones which unionists are most likely to retain anyway.
The SDLP already strongly outpolls Sinn Fein in these three constituencies. In South Belfast, for example, it won 22 per cent of the vote in 1998 while Sinn Fein secured only 6 per cent. Sinn Fein has little to lose by standing aside, the SDLP argues.
But the constituencies in which Sinn Fein leads are not so clear-cut. In North Belfast, for example, Sinn Fein is ahead of the SDLP by only 0.3 per cent. In West Tyrone, it leads by 8 per cent but the local SDLP man who contested the last Westminster election is standing aside for the party's high-profile Agriculture Minister, Mrs Brid Rodgers.
The SDLP firmly believes Ms Rodgers can pull back ground from Sinn Fein and take the seat from anti-agreement UUP MP Mr Willie Thompson. In other constituencies the SDLP is not anxious to be given a free run. For example, if Sinn Fein stood aside, it could possibly win Upper Bann, where Mr David Trimble is the sitting MP. But the SDLP knows this would be disastrous for the peace process.
A party source says today's meeting will be a charade: "Sinn Fein might say that everything is on the table but in reality they will be offering us a deal they know we can't accept.
"They want to go back to the electorate and say `we tried to compromise but the SDLP wouldn't play ball.' " Privately, a Sinn Fein source agrees it is all about PR.
"Talking about a coordinated approach with the SDLP is window-dressing. Sinn Fein aims to overtake the SDLP. That's the talk at all internal election meetings."
He says Sinn Fein had believed it would win West Tyrone but is now "very concerned" about Mrs Rodgers's challenge.
In the event of no Sinn Fein-SDLP pact, the only other seat nationalists have a good chance of taking from unionists is Fermanagh and South Tyrone.
If, as appears increasingly likely, the DUP stands against pro-agreement Ulster Unionist candidate Mr James Cooper, then Sinn Fein's Ms Michelle Gildernew would be the hot favourite to take the seat.