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SEATTLE -- The Seahawks braced for another rampage from Frank Gore. Instead, they got a career passing day from Alex Smith, the 49ers' new spread offense -- and a gift win.

"It's a load off when they take care of one of our problems for us. It was ... beautiful," Seattle rookie outside linebacker Aaron Curry said, beaming that the same Gore who had romped over the Seahawks for 202 yards three months ago in San Francisco ran just nine times Sunday.

That and an avalanche of 49ers mistakes kept Seattle in the game. Then Olindo Mare kicked a 30-yard field goal as time expired to send the Seahawks to a 20-17 victory over San Francisco.

"It's disheartening right now. It's sick," 49ers linebacker Patrick Willis said.

After the 49ers called timeout to try to ice the veteran on an already cold day, Mare made his second go-ahead field goal of the final quarter and extended his team record of 18 consecutive makes for the Seahawks (5-7). They kept alive hopes for a winning season, if not a playoff berth.

The 49ers (5-7) blew more chances than they could have dreamed of. Not the least of which: the opportunity to keep pressure on Arizona in the NFC West. The Cardinals come to San Francisco next Monday night.

"In every situation, we just found a way to screw it up," 49ers coach Mike Singletary said. "Teams that talk about going to the playoffs can't do that."

Smith, who didn't have a 300-yard passing day in his first 38 NFL games, threw for 310 yards in the 49ers' new pass-happy offense. The No. 1 overall pick in 2005, reinstalled as San Francisco's starter in October, completed 27 of 45 throws and had two touchdown passes.

One was to Vernon Davis, who had six catches for a career-high 111 yards. Davis' 10th touchdown of the season set a single-season record for a 49ers tight end.

Yet San Francisco regretted all its blown chances:

• Incompletions on third and fourth downs from the Seattle 1 in the opening quarter.

• A fumble on a punt-return reverse botched by Arnaz Battle and Brandon Jones.

• Another fumble by Gore, when the 49ers finally dusted off the mothballed runner. It was forced by Jordan Babineaux at the Seattle 22 and returned 43 yards by Josh Wilson to set up Mare's first field goal of the final quarter.

• Davis having Smith's would-be touchdown pass zip through his hands and off his facemask in the end zone while down 17-14 late in the game.

"I didn't know who he was throwing to," Davis said.

Then, rookie Michael Crabtree wasn't looking as the ball grazed off the top of his helmet in the end zone, after a blitz forced Smith to throw earlier than Crabtree expected. The 49ers settled for Joe Nedney's 34-yard field goal and a tie with 2:57 remaining.

San Francisco learned its pass-heavy approach has its drawbacks -- unless you are a Seahawk dreading the task of stopping Gore.

He had averaged 160 yards from scrimmage in his previous seven games against Seattle. His career high of 212 yards in 2006? Yep, that came against the Seahawks.

"Any time you play a team that rushes for almost 300 yards, why not do that again -- or at least attempt to?" Curry said.

Seahawks coach Jim Mora said of Gore: "I'm glad he didn't have the ball more."

The final quarter was full of misfires, epitomizing the mild, mild West.

Four consecutive completions by Matt Hasselbeck, three to T.J. Houshmandzadeh, got Seattle to the San Francisco 40 with 1:19 left. Then came the game's fourth negative-yardage completion, a screen pass to Julius Jones destroyed by Willis for a 5-yard loss.

A false start by center Chris Spencer pushed Seattle into a third-and-19 with 1:03 left. Hasselbeck lofted a perfect throw in stride into Deion Branch's arms at the 49ers 12, but the ball skidded through his hands.

Hasselbeck bent over in disbelief, and Seattle punted for the 10th time.

But the 49ers went three-passes-and-out from their own 11. Seattle safety Deon Grant dropped the last throw at midfield.

Nate Burleson returned the ensuing punt 21 yards to the Seattle 48 with 21 seconds to go. Two plays later, rookie Deon Butler ran past cornerback Keith Smith and Hasselbeck's laser pass hit him in stride for a 32-yard gain.

Hasselbeck said Butler isn't even one of the first five options on what the Seahawks call an "alert" play. As Hasselbeck explained it: "Hey, listen, you'll probably never get this ball, but be alert."

But when the 49ers cheated a safety to the other side of the field where three wide receivers were and Smith played up on Butler, Hasselbeck knew Butler was his choice.

One run later, Seattle called timeout, and Mare calmly did his job. Holder Jon Ryan and then the rest of the Seahawks mobbed him on the field.

The oldest Seahawk just shrugged.

"I get paid to make field goals," the 36-year-old said.

Game notes

Smith had 183 yards midway through the second quarter as San Francisco threw the ball on 21 of its first 24 plays. ... Mare has made 21 of 23 field goals this season. He is 45 for 50 since joining Seattle before 2008.