The contest in East Derry is between two well-known anti-agreement candidates. At the last election the UUP's Mr William Ross beat Mr Gregory Campbell of the DUP by almost 4,000 votes. This time around East Derry is one of the key seats being targeted by the DUP.
Mr Campbell has a lot in his favour. He had only been in the constituency for 23 days before the last Westminster election and he topped the poll in the Assembly elections.
His main constituency office is in Coleraine, closer to the bulk of unionist voters than Mr Ross's facility in Dungiven.
His profile has also risen dramatically since the last election, first as the party's security spokesman, then one of its leading performers in the Assembly and now as Minister for Regional Development.
Mr Ross is far from finished. He is the sitting MP and has been one of the party's most uncompromising critics of the agreement. He has been an MP since 1974 and voting for him is a tradition with which many, especially those wary of the DUP, will not want to break.
Perhaps wary of being too closely associated with the UUP's current policies, Mr Ross has campaigned in a semidetached manner; when Mr Trimble came to Coleraine last week, the sitting MP was absent. It is also mathematically possible that the SDLP candidate, Mr John Dallat, could sneak in between the two unionists if they split the unionist vote evenly enough.
There are two borough councils in the constituency. In Coleraine, any slippage of the UUP's position would be interpreted as a falling away of support for the Belfast Agreement. The party took 10 seats at the last elections in 1997, twice as many as its nearest rival, the DUP.
In Limavady, where there is a slim nationalist majority, Sinn Fein has made gains in recent years but the SDLP is still the dominant nationalist party.