A small army of reporters has mobilized in the foothills west of Denver to hear Sen. Gary Hart (D-Colo.) today announce what is expected to be his decision to step down from the Senate, paving the way for another presidential run in 1988.

Political professionals in Washington and Colorado seem unanimous in the belief that Hart will not seek a third term in the Senate. Whether he will formally declare his presidential candidacy today is uncertain.

The federal election law gives Hart a good reason to avoid declaring his presidential candidacy this early. An undeclared candidate has much broader fund-raising and spending options than a formally declared one.

Hart and his staff have made the most of today's announcement, with the inadvertent help of Sen. Edward M. Kennedy's decision to take himself out of 1988 presidential consideration two weeks ago, leaving Hart the apparent front-runner. Four major television networks, at least a dozen major daily newspapers and newspaper groups, two national news magazines and two dozen Colorado newspapers, among others, will witness today's announcement, which is generally viewed as a move preliminary to a second presidential race.

Hart has been teasing the media and the political community about his intentions since the end of the 1984 presidential campaign. In preparation for today's event he began offering himself to the networks and major newspapers for interviews at least two or three weeks before Kennedy's announcement on Dec. 19, part of the media planning necessary to a candidate who needs to remain a national figure in the very early stages of a presidential campaign.

His original plan was not to have a news conference but to have a meeting with a small group of friends and supporters and give a series of private interviews outlining his plans to political reporters at his cabin on Troublesome Gulch Road outside Denver.

The Hart camp rejected this plan when it became clear that it did not suit the schedules of the television networks, the daily press or the weekly news magazines. The Colorado newspapers also objected to what they regarded as preferential treatment for the national media.

Hart granted NBC an interview at his cabin on Thursday and was interviewed yesterday by CBS, CNN, Time and Newsweek for weekend stories, all the time remaining coy about his actual plans for 1986 and 1988. ABC broadcast a profile of Hart on its evening news program last night, including an interview.

The Washington Post and The New York Times were scheduled, as of last night, for a joint interview with Hart this afternoon after his news conference and the rest of the newspapers are scheduled to meet jointly with him on Sunday morning.

Hart's courtship of the media for this event actually began last August when he commissioned a poll by Democratic pollster Peter D. Hart which showed that Coloradans were more favorable and fewer had negative feeling about his 1984 presidential race than a Republican poll.

Colorado state Republican chairman Howard H. (Bo) Callaway was determined that Hart and the Democrats would not be the only ones to benefit from the invading media army. He found out which hotel the reporters were staying in and scheduled a press breakfast this morning "to give the out-of-town guys the Republican point of view."