Most Americans have never heard of Rep. Dan Rostenkowski of Chicago, the son and grandson of Cook County ward leaders in what used to be one of the nation's largest Polish-American communities.

Like those of many veteran members of Congress, the 50-year-old Rostenkowski's 20-year career has always been hidden in the anonymity that cloaks those in so large a body as the 435-member House.

But Rosenkowski seems destined to emerge from those shadows. Not only is his influence already considerable, but he is in a position to make himself a power in the House and possibly on the national political scene.

For instance, Rostenkowski right now is the Democrats' chief deputy whip, with a large part of the responsibility for rounding up votes on major House legislation.

He is the head of the Illinois Democrats in the House, an important and sometimes pivotal voting bloc. He is a power in the Cook County machine, which, thought it has lost prestige since the death of Chicago Mayor Richard J. Daley, is still an important political organization.

As chairman of the Ways and Means subcommittee on health, he is a shaper of the nation's health policies - hospital cost containment is in his purview, and eventually national health insurance will be.

He is a member of the ad hoc energy committee, has been instrumental in rounding up votes to pass controversial portions of the energy bill in the House, and he serves as a go-between for the committee, the speaker, and Ways and Means Chairman Al Ullman (Ore.).

Moreover, Rostenkowski is a man with three important options for the future. As next in line to Ullman, he at the least is likely to become chairman of the all-important Ways and Means Committee. As a man on the leadership ladder, he is often mentioned along with Whip John Brademas (Ind.) and Caucus Chairman Tom Foley (Wash.) as a possible contender for majority leader, when Speaker Thomas P. (Tip) O'Neill Jr. (Mass.) retires. And finally, he could probably run for mayor of Chicago, if he ever decided to pick up that option.

Perhaps Rostenkowski's greatest present asset is an uncanny ability to read the mood of the House for his close friend Tip O'Neill.

That ability flows first from the street smarts acquired as a Cook County pol, and second from his reputation as a member's member - protective of his colleagues, sympathetic to revelations of the political pressures they're under.

Finally, Rostenkowski is a character. Physically imposing at 6 feet 4 inches and over 200 pounds, he is blunt and outspoken, holding nothing back. His views are delivered with an arm-waving, table-pounding force that can blow the timid away. Yet there is an unspoken understanding that "the bear is a gentle one. He just plays politics with a Polish passion," as one member said.

Jimmy Carter is one who found out about Rostenkowski the hard way. Early in the presidency, Rostenkowski invited Carter to address an annual Cook County dinner, an important event since it occurred just after Daley's death and the organization wanted to prove it had not lost its muscle though it had lost its leader.

Not only did the White House turn Rostenkowski down, he heard about the refusal from a reporter. "Peanuts" was one of the kinder and more printable things Rostenkowski called the president in accounts that wound up in the Chicago papers and on the president's desk.

"I figured it was downhill from there," Rostenkowski said, and he has continued to pull no punches when giving Carter his assessment of the prospects of a number of presidential programs. "Tip will say, 'We'll try' and Danny will say, 'It won't work,'" White House congressional liaison man Bill Cable said.

Rostenkowski does the same thing with the speaker and the majority leader. But indications are that his assessments are increasingly being listened to, largely because he has been right so often.

Until recently, reading the House was a throwaway talent. Strong chairmen wrote legislation, politics followed party lines and voting was relatively predictable. But with the decline of parties, the independence of younger members and the uncertainty of the times, the mood often is mercurial, bending with every breeze blowing in the land, and Rostenkowski's mood-reading ability is becoming more important.

If Rostenkowski has a shortcoming, it is that he is better at knowing what the House won't do than working toward what it will.

He is faulted for being a naysayer who carps and complains but doesn't work hard enough to get tough legislation passed. "If Danny is interested, he'll work. If not, he won't," a source said. The attitude leaves both the whip system and the speaker short occasionally, better at compiling information than getting votes to pass bills.

Rostenkowski for many years was considered lazy, labeled a House errand boy for Mayor Daley, and dismissed.

He didn't start out that way. When he was first elected to Congress in 1958 at the age of 30, Rostenkowski looked like a comer. Within eight years he was elected Democratic Caucus chairman.

The roof started falling in with the turbulent Democratic convention in Chicago in 1968, the year of the riots outside and turmoil inside the hall. Carl Albert (D-Okla.) was chairing the convention. Rostenkowski was manning the platform phones when President Johnson called, screaming that Albert was losing control of the convention. "Get that SOB out of there! Take the gavel away from him!" LBJ told Rostenkowski.

Albert resented Rostenkowski for that. Also, Mayor Daley, remembering Albert's hesitant performance, told Rostenkowski to withhold the support of the Illinois delegation when Albert was running for speaker in 1970.

Though Rostenkowski was a key in electing Hale Boggs (La.) majority leader, when Boggs tried to appoint Rostenkowski whip Albert vetoed the appointment and picked O'Neill instead. Moreover, Rostenkowski was unseated as caucus chairman by a man he didn't even know was in the race, Olin E. Teague of Texas.

Rostenkowski went into a slump. "He was in a rotten mood. He lost interest," his aide Jim Healey said. He began to skip meetings of the Ways and Means Committee, turning his proxy over to Wilbur Mills.

It wasn't until 1976 that Rostenknowski roused from the slump. Mills by then had plunged into the Tidal Basin and alcohol, and plunged the Ways and Means Committee into trouble.

Democratic liberals had taken the opportunity to expand the committee, pack it and strip it of its power to make committee assignments for Democrats. Al Ullman took over a much chastised panel and Sen. Russell B. Long (La.), chairman of the Senate Finance Committee, was the first to scent the weakness. He began pushing the House Committee around in House-Senate conferences in a way he could not do when Mills was chairman.

That did it for Rostenkowski. During a 1976 tax bill conference he got mad, and became determined to stop the beating. "I stayed up until 4 a.m. studying and working my head off," Rostenkowski said. next day, he pounded the table, took on Long and Sen. Lloyd M. Bentsen (D-Tex.) and beat them on several major points.

In the same year, Rostenkowski decided to try his hand at kingmaking again. He urged Jim Wright (Tex.) to enter the majority leader fight between two liberals, Richard Bolling (Mo.) and Phillip Burton (Calif.).

Wright beat Burton by one vote, and appointed Rostenkowski chief deputy whip.

Then Mayor Daley died. "Daley's va said. He became his own man, death freed Danny," Rep Abner Mikva.

By nature, Rostenkowski is more conservative than many of the younger members. A graduate of a military academy, he who always wears a sometimes lectures younger members on their casual dress and deportment. "He's always is telling me to button up my top shirt button and tighten my tie," Rep. Marty Russo (Ill.) said.

He also votes more conservatively, particularly on "reform" matters dear to the younger members. "Reform is an obnoxious word to Danny," Mikva said."He doesn't understand an institution must change or die."

But in his fiscal conservatism Rostenkowski finds the House moving toward him. Now he and Ullman do a sort of good cop-bad-cop act. Ullman talks compromise, Rostenkowski takes the hard line.

Rostenkowski recently went with Ullman to tell the president that tax reform was dead and the tax cut was shrinking. "President's don't always get what's right. Next time he should try trimming the hedges instead of chopping down the bush," Rostenkowski said.

Rostenkowski thinks that the president is learning about Congress. "I think that at first he thought he could simply propose something and it would be passed because he was president. It just doesn't work that way."

But he worries about and has warned the president about a "mood of indifference" in the House. "If we win there's no joy. If we lose there's no sorrow. The only excitement comes among the Republicans when they beat us."

Whatever else, Rostenkowski is no longer indifferent.