President Carter's popularity continues to decline and he is now in danger of running third behind Republican Ronald Reagan -- who has a wide lead -- and independent John B. Anderson in the November election, according to two new national polls.

The latest Gallup Poll shows Carter's popularity at 21 percent, the lowest recorded for any president since Gallup started measuring presidential popularity more than 40 years ago. An ABC News-Harris Survey released last week also found Carter's popularity rating at an alltime all-time low.

A Harris Survey released yesterday (a companion of that earier poll) shows Carter running behind Reagan and Anderson in a three-way trial heat.

Harris surveyed 1,458 likely voters from July 18 to 21, and asked them their preference in the presidential race. The figures were Reagan 53 percent, Carter 26 and Anderson 18. But in a trial heat that assumed Anderson had a chance to win, the figures were Reagan 49 percent, Anderson 25 and Carter 23.

Carter ran last in the East, Midwest and the West, according to the Harris Survey.

The Gallup Poll said Carter's popularity is now 7 percentage points below his previous low -- recorded exactly a year ago -- and 40 points below his rating in early December, a month after the Iranian hostage crisis began.

The previous all-time lows in popularity for presidents were 23 percent for Harry S. Truman in November 1951 and 24 percent for Richard M. Nixon shortly before he resigned in August 1974.