United midfielder Dwayne De Rosario, right, was ejected after this tussle with former teammate Danny Cruz of the Philadelphia Union during a preseason game in Orlando on Feb. 23. The teams square off on Sunday in Washington. (Al Messerschmidt/Getty Images)

The last time they met in the regular season, at the peak of the 2012 MLS playoff chase, neither D.C. United nor the Philadelphia Union received a red or yellow card. And aside from United’s Lionard Pajoy scoring against his former team at PPL Park, the midweek match tendered few memories.

It was as if the sides had agreed to catch their breath and set aside differences that had escalated rapidly since the Union joined the league in 2010.

Up to that point, in both MLS and tournament competition, the clubs had packed enough disorder to fill a rivalry dating back decades.

The first formal collision of 2013 comes at 5 p.m. Sunday at RFK Stadium, but the prospects of another peaceful, humdrum affair seem remote.

The preseason finale two months ago rekindled the animosity as a sour-tempered match gradually unraveled, culminating with United captain Dwayne De Rosario head-butting ex-teammate Danny Cruz on the nose.

With United (1-4-1) starved for points, the temperature remains up.

“It has always been ugly between us,” United defender Brandon McDonald said. “We’re two grinding teams. It’s not a bunch of superstars. We are hardworking people, and when you put two teams like that together on the field, it is bound to happen.”

In seven regular season meetings, the sides have combined for 23 yellow cards and four red cards. Another 10 yellows and three reds were shown in two U.S. Open Cup matches. And the preseason game in Orlando featured three yellows and two reds.

It hasn’t been about just cards. Peter Nowak, then the Union’s coach, was tossed from the 2011 Open Cup game — one in which both teams scored in overtime and United won in a shootout. Last year’s cup game was decided in overtime, with onetime United prodigy Freddy Adu setting up the winning goal for the Union.

A regular season match last August was marred by three ejections after the 89th minute, a missed penalty kick, a series of controversial calls and biting postgame comments.

Two games have ended 3-2, both won by Philadelphia, and two others were 2-2.

“I’m not going to lie — I certainly want to go out and win this game more than I have the last four or five games” this year, Cruz told the Union’s Web site.

Cruz and De Rosario have spoken privately since the incident, which resulted in a two-game suspension for De Rosario.

“That’s done,” De Rosario said, declining to elaborate on the conversation.

“He really is a class guy on and off the field and it was just something that happened in the heat of the moment,” said Cruz, who was traded by United last summer for Pajoy.

For United, games against Philadelphia don’t stir emotions like ones with the New York Red Bulls, who, like D.C., are MLS founding members. But they are getting close. Proximity is a factor. The stadiums sit 119 miles apart, the closest MLS venues other than PPL Park and Red Bull Arena at 107 miles. (The Los Angeles Galaxy and Chivas USA share Home Depot Center.)

Hundreds of United and Union supporters typically travel to matches between the clubs.

Familiarity adds to the fervor: Three Philadelphia players, including captain Brian Carroll, were once with United and two D.C. players were with the Union (2-2-2). In addition, several players have crossed paths on junior national teams and in college.

“I don’t like Philadelphia, to be honest with you,” United goalkeeper Bill Hamid said.

While United could use acrimony as motivation, the club can’t allow it to distract from the urgent need for victory. D.C. has scored just two goals in 540 minutes and been shut out four times.

“I want to end the game with 11 players,” midfielder Chris Pontius said.

“We’re going to win. We’ve made it our lives” since losing at home to New York last week, Hamid said. “Three points is what we need and three points is what we are going to get.”