Lopez and Verdasco defeated David Nalbandian and Agustin Calleri 5-7, 7-5, 7-6 (5), 6-3, boosting Spain's hopes of winning the best-of-five championship on the road despite the absence of an ailing Rafael Nadal.
Spain could be in good position for its third Davis Cup crown if Argentina's leading player, Juan Martin Del Potro, cannot play reverse singles Sunday because of injury.
"This was an important victory; it gives us life," Spain captain Emilio Sanchez Vicario said. "We need one more game."
Sunday's schedule tentatively has Del Potro meeting David Ferrer, then Nalbandian playing Lopez at Islas Malvinas Stadium. On Friday, Nalbandian defeated Ferrer in straight sets and Del Potro fell to Lopez in four.
"We have to keep fighting, there are still two points to play," Argentina coach Alberto Mancini said. "We know it's hard, but we can do it. We have to overcome this difficult moment and get our hopes up."
Mancini said his choices for Sunday will depend in part on Del Potro's injury. Sanchez Vicario said he was expected to stick with Ferrer and Lopez.
Nalbandian left the stadium without talking to reporters. Mancini said he was not feeling well, but discounted any injuries.
Spain has never won the Davis Cup away from home. Argentina, unbeaten at home since 1998, has to win both for its first Davis Cup triumph. But its hopes became bleaker Saturday, with Del Potro undergoing treatment on his right thigh after hurting it in Friday's singles. If he cannot play, he could be replaced by Calleri or Jose Acasuso.
"It's not over yet," Calleri said. "We are not happy right now because we wanted a victory, but we still have chances tomorrow [Sunday] and we remain confident."
Calleri and Nalbandian, playing their first Davis Cup doubles since the 2006 final, won the first set, but couldn't hold back the veteran Spanish pair in a match that lasted 3 hours, 18 minutes.
The Spaniards had 86 winners and only 30 unforced errors. The Argentines struggled on serve and were broken six times. But they came through first, breaking Verdasco for 6-5. then erasing a 0-40 deficit for Nalbandian to hold and win the first set.
The rally pumped up the crowd, and the nearly 10,000 fans chanted constantly between games. A small group of Spaniards tried to keep up, at times managing to make some noise.
Argentina saved three break points on Nalbandian's serve in the eighth game of the second set to stay even at 4-4. But after Spain went up 6-5, Nalbandian could not hold again. Calleri's backhand volley flew long to even the set score.
In the third set, Nalbandian and Calleri were broken in successive games to trail 5-1, but Argentina came back to make it 5-5.
With the crowd growing louder, the Argentines seemed ready to prevail in the tiebreaker after jumping ahead 4-0. But Spain won six straight points to win the tiebreaker 7-5 and hush the crowd.
"The third set was full of different emotions," Sanchez Vicario said. "It was an incredible finish and gave us confidence to keep going."
The match was stopped for a few minutes during the tiebreaker because of fans from both nations provoking each other and making noise during points. Sanchez Vicario later said his players were insulted during the match.
In the final set, the Spaniards bolted to a 5-2 lead and cruised to their fourth win in seven Davis Cup doubles, and by far their most important.
"These are very special matches, when you are playing for your country," Lopez said. "I didn't expect to win yesterday and didn't expect to win today."