Labour's Ian Austin holds Dudley North with the slenderest majority of any constituency in the Black Country.
But a poll by Tory peer Lord Ashcroft has shown it is just barely ahead of UKIP while the Conservative vote has fallen away.
The seat is a crucial target for Labour and the Conservatives if either party is to govern with a majority following the General Election in May 2015.
UKIP candidate Bill Etheridge today said the result vindicated his claim that it was a 'two horse race' between himself and Mr Austin. Mr Austin said the only poll that mattered was next May.
The Conservatives came close to taking Dudley North in 2010 and won other seats from Labour including Dudley South, Halesowen and Rowley Regis, Stourbridge, Wolverhampton South West, Cannock Chase and Stafford.
UKIP came a distant fourth in 2010 with just 3,267 votes in Dudley North.
That appears very different now according to Lord Ashcroft's poll of 1,001 voters.
Asked who they would vote for if there was a General Election the next day, 253 said Labour, 237 said UKIP, 185 said Conservative and 18 said Green.
There was also bad news for the Liberal Democrats, who received the backing of just 16 voters in the sample. They came third in 2010.
There were seven people who said they would vote for 'other' parties with the rest either undecided, not planning to vote or refusing to say.
Lord Ashcroft said: "On the Labour-UKIP battleground I looked at four constituencies in which, according to the Fabian Society's Revolt On The Left report, Labour are at 'critical' or 'high' risk of a direct challenge from UKIP: Great Grimsby, Dudley North, Plymouth Moor View and Rother Valley.
"I found Labour ahead – just – in all four of these seats.
"But if the Labour Party is worried about them, it has good reason to be. In three of the seats I found UKIP ahead by up to six points on the standard voting intention question, and they were tied with Labour in Dudley. It was only when people were asked how they would vote in their own constituency that Labour's lead reasserted itself: six points in Rother Valley, five points in Plymouth Moor View, three points in Dudley North and one point in Great Grimsby. In the seats as a whole the swing to UKIP was 13.5 per cent."