Ben Roethlisberger exchanged high-fives with teammates after the touchdown that extended the Pittsburgh Steelers’ lead in the fourth quarter.

With his left hand. While wearing a baseball cap.

With Roethlisberger sidelined after reinjuring his throwing shoulder, the local kid who used to sneak into Washington Redskins games made an unexpected appearance on the field.

D.C. native Byron Leftwich came on at halftime and led two touchdown drives in the Steelers’ 23-6 victory over the Redskins on Monday night.

‘‘We all know Ben’s the guy,’’ Leftwich said. ‘‘Ben’s the quarterback of this football team, and I know I was going to be there in case something happened. Something happened today.’’

What happened was Roethlisberger’s 1-yard sneak that gave the Steelers a 10-6 lead in the final minute of the first half. On the play, the quarterback reaggravated the slightly separated right shoulder that has bothered him since the second week of the season.

‘‘I have no update on Ben at this point,’’ coach Mike Tomlin said. ‘‘Potentially, he was capable of going back in the game, but we went down the field with Leftwich in there to start the second half. We’ll just see how he is.’’

Roethlisberger was 5-of-17 for 50 yards and an interception, while Leftwich went 7-of-10 for 129 yards and a touchdown as Pittsburgh (6-2) beat an NFC East team for the first time in three attempts this season. Leftwich had 44 friends and relatives at the game — all of them Redskins fans growing up — but he said jokingly that ‘‘none of them better be upset’’ after the Steelers handed Washington (6-3) its worst loss of the season.

‘‘I truly don’t know the offense yet,’’ said Leftwich, who signed with the Steelers in August. ‘‘I just got here late in the preseason, so that’s what makes it tough. … That’s why I came out so early during halftime. It’s like, ’If I’m going to play, I’ve got to get loose.’’’

The Steelers’ top-rated defense had seven sacks and became the first team this season to intercept Jason Campbell. It also held Clinton Portis to 51 yards rushing, ending his streak of five straight games with at least 120 yards.

‘‘We were hoping to go out and play our football, play smash-mouth football,’’ Portis said. ‘‘Instead, we got smashed.’’

While the Steelers remain atop the AFC North, the Redskins lost ground to the New York Giants in the NFC East. Washington enters its bye week needing to tweak an offense that puts together promising drives but struggles to find the end zone.

The Redskins came close to scoring a touchdown only once — when Campbell threw incomplete on fourth-and-goal at the 1 in the fourth quarter. Campbell went 24-of-43 for 206 yards and his streak without an interception reached 271 attempts — 249 this season — before cornerback Deshea Townsend grabbed a pass tipped by Portis late in the third quarter. Campbell, on the run for much of the game, threw another interception in the fourth quarter.

‘‘When the team that they’re playing has to throw, they just kind of put the warrior bandanna on — and here they come,’’ Redskins coach Jim Zorn said of the Steelers pass rush.

The game was the first hosted by Washington on the eve of a presidential election since 1984, and there was no mistaking the combination of football and election fever. One fan alternately waved a white towel with Barack Obama’s image in the left hand and an all-burgundy Redskins towel in the right hand. Sports-themed interviews with Obama and John McCain were broadcast by ESPN during halftime.

The Redskins also used the special occasion to wear burgundy jerseys and pants together for the first time in franchise history. A more noticeable color, however, was the yellow from the sea of Terrible Towels waved by Steelers fans who managed to secure tickets by the boatload for the lower bowl of the stadium. Visiting Pittsburgh players were waving to the fans all around to make noise during defensive stands.

‘‘There was a lot of ’em,’’ Campbell said. ‘‘I was definitely surprised to see that many of them in our home stadium.’’

The Redskins were on the board with two field goals in the first four minutes — without the benefit of a first down. They recovered the Steelers surprise onside kick that opened the game, and Cornelius Griffin intercepted a tipped pass. Washington didn’t convert a third down until late in the third quarter.

The Steelers didn’t cross midfield until Carlos Rogers grabbed Hines Ward on an obvious 43-yard pass interference penalty in the second quarter, setting up a field goal that cut Washington’s lead to 6-3.

The Steelers went ahead after Andre Frazier blocked Ryan Plackemeier’s punt. William Gay recovered at the 13, leading to Roethlisberger’s 1-yard score.

Leftwich’s big play was a 50-yard completion to Nate Washington before a 1-yard TD run by Willie Parker, who was back from a four-week layoff with a knee injury. Leftwich also had a 5-yard scoring pass to Santonio Holmes, who returned from a one-game benching over a charge for a marijuana-related offense.

‘‘We didn’t think twice about him coming in and doing the job,’’ Washington said of Leftwich. ‘‘We never skipped a beat.’’

Notes: Other injuries: Pittsburgh TE Heath Miller (ankle) and Redskins DT Anthony Montgomery (strained Achilles). … Campbell’s interception also ended the Redskins’ NFL record of 379 regular-season attempts without one.