Sean Dyche's Burnley side were held 0-0 by playoff chasers Cardiff City as their lead at the top was cut by Boro, who have won their last three and have a game in hand over their rivals in the race to win a place in the Premier League.
Brighton & Hove Albion in third are two points off the pace after coming back from a goal down to beat Birmingham City 2-1, but fourth-placed Hull City's hopes of automatic promotion were hit as they lost 4-0 at Derby County in sixth.
Sheffield Wednesday, who are fifth, strengthened their playoff hopes with a 2-1 home win over Blackburn Rovers.
While the wheels have not come off Burnley's second-tier title challenge, their momentum has certainly stalled with Tuesday's stalemate against Cardiff coming on the back of successive draws with Brighton and Wolverhampton Wanderers.
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
Cardiff keeper David Marshall was in good form, denying Andre Gray and George Boyd as well as Sam Vokes and Ashley Barnes late on as Burnley endured a frustrating evening.
Middlesbrough, who have reeled in Dyche's side over recent weeks, took the lead against Huddersfield when captain Grant Leadbitter fired home a penalty after 32 minutes.
Gaston Ramirez, who won the spot kick, added a second goal moments later before the Uruguayan, on loan from Southampton, rounded off the scoring with a free kick 13 minutes from time.
Brighton, who fell behind when Northern Ireland striker Kyle Lafferty scored for Birmingham, kept up the pressure on their rivals with goals from Connor Goldson and Lewis Dunk.
Hull's hopes, however, took a battering in a comprehensive defeat by Derby that left them eight points adrift of Burnley.
Bradley Johnson scored twice for Derby, before Hull's Moses Odubajo was sent off in the 55th minute. Chris Martin then scored a long-range effort and Craig Bryson tapped in late on.
(Reporting by Toby Davis; editing by Ken Ferris)
