Northern Virginia Republican Rep. Stanford E. Parris began his bid for reelection yesterday with a strong endorsement of President Reagan's policies.

"My name is Stan Parris, I'm a Republican, and I support the president of the United States," he told a district GOP convention that unanimously endorsed his candidacy. The president and Republicans in Congress, Parris continued, "are going to lower interest rates . . . and bring the budget under control and we're going to do it now."

Parris regaled the audience of 300 at Lee High School in Fairfax County with a five-point checklist of the differences between Republicans and liberals. One difference, he said, is that liberals believe individuals "ought to feel sorry" for muggers, while Republicans believe that, "whatever their childhood, if an adult person mugs you, they are a dangerous criminal and ought to be prosecuted." That remark brought the loudest applause of the convention.

Parris, whose 8th Congressional District includes Alexandria, southern Fairfax County, eastern Prince William County and northern Stafford County, was introduced by Fairfax County Board Chairman John F. Herrity, who told the crowd that Parris had rescued them "from the Herb Harris hogwash." Parris' opponent this fall is Democrat Herbert E. Harris, whom Parris ousted from the seat two years ago.

Yesterday's meeting also was a pep rally for Rep. Paul S. Trible, who is expected to receive the Republican nomination for the seat held by retiring Sen. Harry F. Byrd Jr. at the state GOP convention in Richmond June 5. Trible introduced himself as "the only unopposed candidate for the U.S. Senate in America," a reference to the fact that no Virginia Democrats have announced for the seat.