The possibility of a large Democratic crossover vote for Rep. John B. Anderson in Tuesday's Illinois primary appears to be the only thing blocking Ronald Reagan from taking a giant step on the way to his party's presidential nomination.

Reagan is a clear favorite among traditional Republican voters in the state, polls indicate. But Anderson, until recently regarded as a dark horse in the race, runs well among independents and Democrats, and if enough of them vote in the GOP primary, he is expected to pose the former California governor a stiff challenge.

"It's all in the turnout," says Anderson's pollster, Dick Bennett. "We'll get between two-thirds and three-fourths of the Democrats and independents who turn out. The question is how many of them will vote."

A poll in today's Chicago Tribune indicated that one in seven persons who said they would vote in the Republican primary is a Democrat. Anderson's strategists hinge their hopes for a win on attracting enough Democrats and independents to the polls to make up from 20 to 25 percent of the vote.

But surveys by respected private pollsters indicate a likely crossover vote of slightly under 20 percent, which would mean a Reagan victory.

Democrats believe the crossover will cut two ways in their party primaries. They say it threatens to hurt Carter by draining away voters in Chicago, where the Democratic organization is backing Kennedy. But in suburban and downstate areas Anderson will draw liberal votes away from Kennedy.

Chicago, independent Democrats, interested in key local races, have gone so far as to ask followers who support Anderson to write his name in on the Democratic ballot, rather than vote in the GOP primary.

The primary is pivotal for all of the Republican candidates.

For Reagan, a victory in this large, midwestern state would go a long way toward helping him sew up the nomination. For Anderson, his home state provides him a chance to emerge as an alternative to Reagan among party moderates. For George Bush, who's running a weak third in the polls, the primary once offered a chance to check his rapid fall as a serious contender for the GOP nomination -- a chance he apparently has failed to develop.

The fourth candidate in the race, Illinois Congressman Philip M. Crane, is running well behind in the polls and is not considered a serious factor.

"We're going to win tomorrow. We're going to win," Anderson told a chilly, sleet-soaked rally in downtown Chicago at noon. "The people are listening. It's a new message we've given them. It's a different message. But it is the same message the last Illinois president [Abraham Lincoln] took to the White House."

Bush, however, almost conceded defeat today. He said in a television interview that he didn't expect to do well against two native sons. But he also indicated that he's looking forward to the coming primaries in Connecticut and New York and is "in it [the race] to the end."

"I think Ford's pulling out helped me considerably," Bush said. "We've come a long way. No one ever said it would be easy, and it isn't. But we're well-financed, and I am second in the total number of delegates now nationwide."

The Illinois primary comprises two unrelated contests -- a nonbinding "beauty contest" for presidential preference and a selection of delegates to the National Republican Convention. The vote in the beauty contest has no bearing ont he selection of delegates, all of whom will go to the GOP convention technically uncommitted.

There are 344 candidates for Illinois' 92 delegate slots, chosen by congressional districts. About two-thrids of them favor one presidential candidate or another. But it will be virtually impossible for voters to know which candidate the delegates support because, unlike the Democratic primary, no presidential candidate's name will appear on the delegate selection ballot.

Reagan has the most delegate candidates -- 74, according to a tabulation by the state party -- and is favored to win the delegates contest handily. However, uncommitted candidates make up the largest single block of delegates -- 108. This includes "unity" slates, made up of well-known party officials in 10 congressional districts.

Anderson has 53 delegates candidates, Bush 37 and Crane 35. The other delegates prefer other presidential contenders, or their preference is unknown. But state party officials say it will be difficult to tabulate the winnes Tuesday night, and it probably will be Wednesday before it is known who has won the delegate races.

Reagan, like Anderson and Crane, has strong home-state ties to Illinois. He was born in the small farming town of Tampico, went to high school in nearby Dixon and was graduated from tiny Eureka College before going on to become a movie star. Although he narrowly lost the 1976 Illinois primary to Ford, he was considered the early favorite here.

But Anderson, a 10-team congressman from Rockford, shook up party regulars when he emerged from second-place finishes in the Vermont and Massachusetts primaries and took a narrow lead over Reagan early last week in public and private opinion polls.

Anderson has benefited since then from extremely favorable attention in Chicago's two major newspapers, both of which glowingly endorsed him as courageous and independent, and from widespread support from students and independents in the Chicago area.

But polls indicate Reagan began to gain ground last week after he, Bush and Crane attacked Anderson's credentials as a Republican during a televised debate last Thursday. Crane and Reagan went so far as to suggest that Anderson should run as a Democrat.

Even the normally passive Bush has gone on the offensive against Anderson, an avowed moderate. Bush has taken out a blistering series of television advertisements attacking Anderson and was the first to go on the offensive during the debate.

The attacks, however, backfired on Bush, a former ambassador and director of the CIA, according to polls for his opponents. He apparently lost support while Reagan picked it up.

After the debate, Reagan stepped up his attacks on Anderson, suggesting at a news conference last Sunday that the congressman, who has refused to say he would support Reagan in the general election, think about switching parties. Anderson countered that charge today.

Referring to Reagan's timid support of Gerald Ford in the 1976 general election campaign, Anderson said, "If he had been a real Republican in 1976 instead of sitting on his big horse on his ranch, we would have a Republican president today. Who is calling who a real Republican?"

At a rally in New Britain, Conn., tonight, Reagan, asked whether he would take Anderson as his vice presidential running mate, replied: "John has just said there's no way he would support me. I think that anyone who would put him in the second spot [on the ticket] would have to look over his shoulder."