President Carter's reelection campaign will begin unofficially today, when his campaign committee, named the Carter-Mondale Presidential Committee, formally files papers with the Federal Election Commission.

Evan Dobelle, the treasurer of the Democratic National Committee, will be the chairman of the committee, and John Dalton, who recently resinged as the president of the Government National Mortgage Co., will be its treasurer, Dobelle said last night.

He said the committee headquarters near Connecticut Avenue and K Street will open March 26 with a staff of 15 to 20.

Dobelle also named three other officials of the committee. Tim Smith, a former deputy appointments secretary in the White House and former Justice Department lawyer, will be the committee's legal counsel. Lori Baux, deputy director of the White House personnel office, will be Dobelle's executive assistant, and Donna Sagemiller, the controller of the DNC, will be the committee controller.

Although the president has not declared that he will seek reelection, Dobelle said the committee is a fully authorized organization and not one of the so-called "exploratory" political action committees similar to those set up by several potential Republican presidential candidates.

The fact that the committee would be established soon and that Dobelle would head it has been widely reported in recent weeks.

One unusual aspect of the Carter committee is that its official title includes the name of Vie President Mondale. But Carter has said publicly that if he seeks reelection he wants Mondale as his running mate.