The Democratic National Committee's executive board voted unanimously yesterday to crack down on three states that moved up their dates for selecting national convention delegates in defiance of party rules.

"We've got our rules and our responsibilities," committee Chairman Charles T. Manatt said after the vote.

But party chairmen from the three states--New Hampshire, Iowa and Maine--seem to be supported in their defiance by several major presidential candidates.

The national party can set up its system for picking delegates from states that buck the rules. The executive committee authorized Polly Baca Barragan of Colorado to chair such a committee and directed Manatt to appoint other members later.

New Hampshire Democratic Chairman George Bruno is circulating a letter among the presidential candidates that endorses holding caucuses in Iowa on Feb. 20, a primary in New Hampshire on Feb. 28 and caucuses in Maine on March 4, all one week earlier than the rules allow.

Representatives of Democratic presidential candidates Walter F. Mondale, Sen. Alan Cranston of California and Sen. Ernest F. Hollings of South Carolina confirmed that they will sign the letter.

Sen. Gary Hart of Colorado also is expected to sign, a spokesman said.

Former senator George McGovern of South Dakota said in an interview he will not sign, because "I think I need all the time I can get."

Jim Bacchus, spokesman for former Florida governor Reubin Askew, was even more adamant in saying his candidate will not sign.

But Sen. John Glenn of Ohio attempted to go down the middle. He issued a statement saying he would support New Hampshire's early date without saying whether he would sign the letter.