President Carter's pollster and his campaign chairman said yesterday that independent presidential candidate John B. Anderson is suffering a steady decline in support and will soon be the choice of less than 10 percent of the electorate.

Patrick Caddell, the Carter campaign pollster, said that in the last two weeks Anderson has experienced a "major decline" in the suburbs of northern industrial states, dropping 7 to 9 points in some areas. The major beneficiary of this erosion in Anderson support has been Republican nominee Ronald Reagan, who has picked up 2 to 3 points in a band of states from New Jersey to Illinois, according to Caddell.

"I expect the erosion to continue," Caddel said. "Two-thirds of Anderson's voters say they know he can't win, and that will contribute to further erosion. He will be down in the single digits."

Both Caddell and Carter campaign chairman Robert S. Strauss said Anderson appeared to have gained nothing from his appearance with Reagan in a nationally televised debate Sept. 21. But Zev Furst of the Anderson campaign took issue with this and with Caddell's other findings.

"Our figures don't show that," Furst said when told of Caddell's claim that Anderson is in decline. "Our figures show he's held. He did not go up after the debate because we were not able to go into a heavy media buy."

Furst conceded that without a major infusion of money for its media budget the Anderson campaign will face a difficult task in rising above the about 15 percent level of support Anderson enjoyed around the time of the debate. But he accused Carter campaign officials of increasing their efforts to "push aside" Anderson's candidacy to mask the president's failure to make gains in the polls.

Caddell said Anderson is trying to improve his standing by emphasizing his liberal stands on some issues, which is "pushing off his Republican supporters" and benefiting Reagan. Strauss accused the Illinois congressman of "sharpening his attacks on Carter" and said that to counter it the Carter campaign has begun a series of anti-American radio commercials stressing that "a vote for Anderson is a vote for Reagan."

Echoing Caddell, Strauss said the Carter campaign began using the anti-Anderson commercials earlier than planned because Anderson's decline in support was "deeper and sharper" than they had expected.

Strauss also complained about the power in the election of a number of conservative, so-called independent committees that are supporting Reagan. Citing newspaper clippings for his figures, he handed out a document alleging that these groups will spend about $18 million advancing Reagan's campaign.

Strauss said that in recent days the Carter campaign has increased its media budget at the expense of some field operations such as voter registration, but that the independent committees -- which are sanctioned by federal election laws -- would still spend more on media boosting Reagan than the Carter reelection organization does for the president.

Caddell, in his session with reporters, also said Carter is "slightly behind" Reagan in electoral votes at this point in the campaign, "but not as far behind as some surveys show."

He said Carter's "lead is expanding very steadily" in New York but is "a litle lagging" in Ohio. He said the president appears to have improved his position in Pennsylvania and such southern states as Mississippi, South Carolina and Louisiana. Texas is close but "still tough" for Carter, while "we have a real shot at Oregon and Washington," Caddell said.