"I say what I mean, and I mean what I say," Mr Thompson told The Irish Times in April last year. Even if that effectively entailed calling for the resignation of Mr David Trimble and lining up with his bitter unionist opponents, the Rev Ian Paisley and Mr Robert McCartney. Last May he was elected for the first time as an MP in West Tyrone. Sinn Fein and the SDLP split the majority nationalist vote, while the DUP grudgingly stood aside in favour of Mr Thompson.
The UUP deputy leader, Mr John Taylor, accused Mr Thompson of ingratitude yesterday, arguing that but for the resolution of Mr Trimble the DUP would never have agreed to give him a free run in West Tyrone. "Anybody but Willie," was the DUP view at the time. Mr Thompson disputes Mr Taylor's account, arguing that Mr Trimble really wanted a different candidate. He'd certainly prefer one now.
It's ironic that Mr Thompson is currently in the same camp as the DUP because for such a staunch unionist he was almost responsible for handing the Mid-Ulster seat to Mr Danny Morrison, a former Sinn Fein publicity director who served a period in jail for IRA activity.
In 1983 Mr Thompson broke what the DUP was sure was an understanding it had with the UUP to allow the Rev William McCrea to campaign as the sole unionist candidate in Mid-Ulster. Mr McCrea won the seat by the skin of his teeth, polling 16,174 votes, only 78 votes more than Mr Morrison.
Mr Thompson polled a modest 7,066 votes, but sufficient - if he had not broken the UUP-DUP pact - to ensure that Mr McCrea need not have had so many sleepless nights in the run-up to counting day. The DUP, with Mr Thompson firmly in its No camp, now may have partly forgiven him, although this may be simply strategic clemency.
Mr Thompson (58) has a television repair business in Beragh, Co Tyrone. He is a Methodist lay preacher who hitherto was fairly popular among Tyrone nationalists. Even during the height of the 1997 general election a local republican conceded, "He's a unionist, but he's no bogey man."
That has changed. His current stance has dramatically reduced the chances of his holding the West Tyrone seat next time around. Barring unforeseen circumstances, the SDLP and Sinn Fein will be inclined in the next Westminster battle to ensure there is no split nationalist vote, and no more Mr Thompson as their MP.
Not that that will devastate Mr Thompson. As Mr Trimble, pro-agreement unionists and nationalists realise, he's his own man.