Filing of delegates for the key March 18 Illinois primary began today with fresh embarrassments for Sen. Edward M. Kennedy (D-Mass.) and a challenge to the Republican state organization's uncommitted slates from backers of four GOP presidential hopefuls.

Supporters of native son Ronald Reagan, Sen. Howard H. Baker Jr. of Tennessee and Illinois' own hopefuls, Reps. Philip M. Crane and John B. tAnderson, each filed for at least 25 of the 92 Republican district delegate spots, in many cases challenging "consensus" slates backed by Illinois Gov. James R. Thompson and the state GOP organization.

Three others who have filed in the Republican race -- Texans George Bush and John B. Connally and Sen. Bob. Dole of Kansas -- filed only a handful of their own supporters.

As the filings opened in Springfield, President Carter's campaign manager, Tim Kraft, flew out from Washington to present the 30,000 names on nominating petitions for the 152 Carter delegates. Informed observers said the Carter Slates appear equal or superior to Kennedy's in most of the 13 suburban and downstate districts and a threat to the Kennedy slates in perhaps four of the 11 Chicago-area districts.

Kennedy -- who bested Carter in a struggle for the support of Chicago Mayor Jane Byrne last fall -- was the early favorite to win the Illinois primary, considered vital because it is the first test in a major industrial state outside the home base of either man. But, as in other states, Kennedy's polls have slipped since the start of the Iranian crisis. Byrne had to use her patronage muscle to keep organization leaders in several city districts from filing uncommitted delegate slates, and the Kennedy slates in those districts lack the well-known names that are ordinarily found there.

Today, Kennedy's forces suffered three additional minor embarrassments. First, they lost a tug-of-war over black state Sen. Charles Chew, who was listed earlier this week as a 1st District delegate on both the Kennedy and Carter slates, but who wound up today with Carter.

Second a last-minute Kennedy slating meeting for the 5th District -- held in the City Hall office of a Byrne loyalist -- was attacked by state Sen. Richard M. Daley, son of the late mayor and a Byrne rival, as "a mockery of party rules." However, the Kennedy slate in the district is considered far stronger than the Carter delegates.

Third, Kennedy delegates were not filed today in the downstate 24th District, home ground of Kennedy's state chairman, Rep. Paul Simon, because of a lack of sufficient petition signatures to withstand challenge from the Carter forces.

Delegates may file for another six days, but only those entered at the start of business today are eligible for the lottery for top ballot position.

Backers for California Gov. Edmund G. (Jerry) Brown Jr. -- who was campaigning here today -- filed no delegates, but said they would do so later this week as signatures are collected.

The makeup of the delegate slates is considered an important element in the outcome, because the Illinois primary is actually 24 separate contests -- electing delegates in each of the 24 districts. There is separate "beauty contest" preference poll, in which all the Republicans and Democrats are entered. It gets headlines but does not determine delegate distribution.

Republicans this year changed their rules -- with Thompson's blessing -- to remove the presidential preference identification from the delegate candidates' names. The Democratic ballot will still list candidate preference.

The GOP's "blind primary" is expected to enhance Thompson's influence on the delegation and thus his chances for a vice presidental nomination. A challenge to the "blind primary" by Reagan's Illinois chairman, state Rep. Don Totten, was thrown out of court last week, but is now being appealed.

The organization's "consensus" slates -- headed by many of the GOP congressmen and such well-known figures as former secretary of defense Donald Rumsfeld -- are generally being accepted by Connally, who was supposedly Thompson's original choice for president, and by Bush who has more recently seemed to draw the governor's favor.

But Reagan, Baker and Crane each filed at least 30 delegates of their own own today, and Anderson almost that many.

The result is a complex set of alignments, with no two districts exactly the same. But in only a handful of districts is the "consensus" slate effectively unchallenged.