WASHINGTON — Senator Kennedy’s diagnosis of a malignant brain tumor has left Congress without its best dealmaker and boldest liberal, a politician known for his staunch positions and willingness to work with right-wing lawmakers.

The Senate opened debate on paying for another year of the Iraq war without the Massachusetts Democrat’s customary roar of outrage.

Just as evident was his absence when President Bush yesterday signed a measure that Mr. Kennedy forged with Republicans to protect people from losing their jobs or health insurance because their genes say they are prone to future illness.

Whenever there was a deal to be made on important legislation, the scion of the famed political family was somewhere nearby despite his reputation as one of the Senate’s last liberal lions.

That willingness to buck his own party and cut deals means that Mr. Kennedy has left his mark on health care, civil rights, welfare, housing, education, foreign affairs, and other issues.

“He has crossed the aisle and sponsored so many legislative enactments,” Senator Specter, a Republican of Pennsylvania, said.

Senator Roberts, a Republican of Kansas, added, “Although it is no surprise there are areas where we have a difference of opinion, there are also many areas where we worked together, particularly on child-care and disability issues.”

In a climate that values party loyalty and making political points more than making laws, there is a dearth of potential stand-ins.

Senator McCain, the presumptive Republican presidential nominee, is perhaps the closest. But his alliances with Democrats on campaign finance, immigration, torture, and other issues has tested the patience of his party’s conservative base.

House Speaker Nancy Pelosi, a Democrat of California, reached out and negotiated a $168 billion plan of tax rebates and other measure with Mr. Bush this year. Rep. Barney Frank, a Democrat of Massachusetts, has put together bipartisan coalitions for advancing measures to save hundreds of thousands of strapped homeowners from foreclosures and secure civil rights for homosexuals.

Senator Graham, a Republican of South Carolina, has cut deals with Democrats on getting conservative judges confirmed, banning torture in interrogating suspected terrorists, getting tougher with China on trade, and expanding health care for members of the National Guard and Reserves.

But none has the reputation and record that Mr. Kennedy has in assembling coalitions and keeping them together.

In 1973, after the Watergate scandal, Mr. Kennedy co-sponsored the first bipartisan campaign finance bill. It established new contribution limits and a public financing provision for presidential elections.

Mr. Kennedy was instrumental in enacting the Americans with Disabilities Act of 1990, the State Children’s Health Insurance Program, and many health care initiatives.

He was an important backer of Title IX, a 1972 amendment requiring colleges and universities to provide equal funding for men’s and women’s athletics.

He has championed minimum wage increases, pushing the most recent effort to raise it from $5.15 to $7.25 a hour by 2009.

Standing at his side for many of his legislative accomplishments is usually a Republican with whom Mr. Kennedy has worked to get results.

“I have held that view because he remains the single most effective member of the Senate if you want to get results,” Mr. McCain said.

“Senator Kennedy enjoys great respect and admiration on this side of the aisle,” the Senate Republican leader, Mitch McConnell of Kentucky, said. “He is, indeed, one of the most important figures to ever serve in this body in our history, and Republican senators recognize that as well.” While not officially a member of Senate leadership, few deals are struck without Mr. Kennedy’s viewpoint being sought. Mr. Kennedy is the second-longest serving member and is not up for election again until 2012.

“Teddy’s been one of those people who just transcends politics,” Senator Dodd, a Democrat of Connecticut, said.

Mr. Kennedy’s absence leaves a hole in Congress and a definite delay for legislation the powerful Senate Health, Education, Labor, and Pensions committee chairman was working on.

Mr. Kennedy’s not finished, lawmakers said, calling his illness merely a setback. “He’s unstoppable,” Senator Kerry, a Democrat of Massachusetts, said.