Ehrhoff scored his ninth of the season with a shot from the right circle as the Canucks were on a power play. Brad Boyes was given a 4-minute penalty for high-sticking penalty 19 seconds into overtime.
"It was a great pass by [Kyle Wellwood] through the seam," Ehrhoff said. "[Ryan Kesler] was making a great screen there [in front] and the goalie couldn't see it. I just had to put it in the corner."
Henrik Sedin tied the game with 1:56 remaining in regulation for the Canucks, 8-2-1 in their last 11 games. Vancouver scored twice in the third period and sent the Blues, who have the NHL's worst home record at 6-13-3, to their fourth consecutive defeat.
"I thought our guys stuck to the principles out there and battled hard to come back," Canucks coach Alain Vigneault said. "We were able to get a power-play goal in overtime 4-on-3."
Daniel and Henrik Sedin each had a goal and an assist, and Mikael Samuelsson also scored for the Canucks, 8-2-1 in their last 11 games. Vancouver defeated the Blues for the first time in three games this season. Roberto Luongo stopped 22 shots.
"Anybody knows how tough it is to come back in this league," Vigneault said. "I know that St. Louis right now is going through a challenging period, but they're still an excellent hockey team.
"To be where we were and come back from a three-goal deficit and win in OT for our confidence and believing in ourselves is real good."
Andy McDonald had a goal and an assist, and Brad Boyes had two assists for St. Louis, 0-3-1 in its last four games.
"They're all difficult," Blues captain Eric Brewer said. "It's certainly a tough stretch for us. At the end of the day, the penalties really took our flow of our game and we were good with it most of the game. They just found a way to get one when they needed one."
Keith Tkachuk scored his 26th career goal against the Canucks and Alex Steen netted his fourth in three games. Chris Mason stopped 26 shots as the Blues fell to 1-7-2 in their last 10 home games.
"We've been here before and they all hurt. I don't qualify losses," Blues coach Andy Murray said. "This hurts just like every loss that we've had."
The Blues took a 1-0 lead during a 5-minute power play when Steen notched his seventh of the season on a one-timer from the slot with 3:05 left in the first.
Canucks right winger Rick Rypien received a match penalty that resulted in the power play for St. Louis after a fight with Cam Janssen. Rypien received the automatic ejection for having tape on his hands below the wrist that cuts or injures an opponent during an altercation. Janssen was bleeding from the nose.
"In my mind, we got put in a very tough position in the first period when we were assessed that five-minute major penalty," Vigneault said. "In my opinion, it was not the right call. The call clearly states that there has to be a cut. There was no cut. It was a broken nose.
"The tape is on [Rypien's] right hand and he threw punches with his left, got him good a couples times and broke his nose. That's not a cut. So I thought the interpretation of that call was wrong."
Tkachuk made it 2-0 at 4:07 of the second with a one-timed shot into an open side after David Perron fought his way past Mitchell.
McDonald gave the Blues a 3-0 lead with 5:30 remaining in the second. Canucks defenseman Sami Salo backchecked on McDonald's breakaway, but the Canucks failed to clear the zone. McDonald slammed in a shot from the left circle.
Daniel Sedin started the Canucks' rally when his shot from behind the Blues' net caromed off the right skate of Roman Polak and off Mason's right skate with 3:51 remaining in the second.
Samuelsson got the Canucks closer when his bad-angle shot from the left circle squirted through Mason at 5:46 of the third period to make it 3-2.
Henrik Sedin tied it with 1:56 left in regulation. Mason failed to cover a loose puck, and Sedin banged it in off an assist from twin brother Daniel.
"We know that their confidence level is not high right now," Ehrhoff said of the Blues. "We know if we can get one, we can push harder and yeah, we got the job done."
Tkachuk has 62 points in 64 career games against Vancouver. ... Canucks D Kevin Bieksa didn't play after injuring his ankle from a skate blade Tuesday in a shootout loss at Phoenix. ... The Blues scored four goals in the first 7:36 of a 6-1 home win over the Canucks on Nov. 10. ... Blues D Erik Johnson returned to the lineup after being a healthy scratch Tuesday. He is pointless in nine games. ... Steen has 10 points in seven games. ... St. Louis is 13-2-4 when leading after two periods. ... The Canucks, who came in third in the league on the power play, were 1 for 8.
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ST. LOUIS - DECEMBER 31: Ty Conklin #29 and Erik Johnson #6 of the St. Louis Blues warm up before a game against...