Senate Democrats plan to push ahead this week with legislation imposing new ethics rules on the Supreme Court in the wake of disclosures about the justices’ travel and outside activities, despite blanket opposition by Republicans who claim the effort is intended to undermine the high court.

The Judiciary Committee is scheduled on Thursday to consider legislation by Senator Sheldon Whitehouse, Democrat of Rhode Island, that would require the Supreme Court to establish a new code of conduct for justices, set firmer ground rules for recusal from cases, create a new investigatory board and promote transparency about ties with those before the court.

Senate Republicans have made it clear they won’t support the legislation, and it has no chance in the G.O.P.-controlled House. But Mr. Whitehouse said he saw the fight over the measure as just the first step, after a string of revelations about undisclosed luxury travel, relationships with affluent Americans and speaking engagements tied to book sales, as well as the shocking leak last year of the court’s decision overturning precedent on abortion rights.

“You have to start somewhere,” said Mr. Whitehouse. He added, “The more information that comes out about the mischief going on at the Supreme Court, the more inevitable it becomes that they come around to agreeing we have to do something. We’re just at the beginning.”

Republicans say the Democratic focus on the court is meant mainly to undercut its legitimacy in retaliation for rulings on abortion, affirmative action and federal regulatory power that Democrats disagree with, but some concede that the justices have exhibited some lapses and that Chief Justice John G. Roberts Jr. should take steps to address those issues.

“There are actually ethics questions legitimately raised that the court needs to to deal with,” Senator Lindsey Graham of South Carolina, the top Republican on the Judiciary Committee, said. “I think Justice Roberts needs to find a way to assure the public about the integrity of the court. I think he’s better able to do that than Congress.”

Lawmakers have been pressing the high court to adopt clearer ethics rules for years, but the effort has gained new momentum from media reports of undisclosed high-end travel by Justices Clarence Thomas and Samuel A. Alito Jr. and courting of the justices by influential conservatives.

The Associated Press weighed in last week with a report on efforts by an aide to Justice Sonia Sotomayor, who was nominated to the court by President Barack Obama, to pressure public institutions hosting the justice to purchase books she has written.

Senator Richard J. Durbin, Democrat of Illinois and chairman of the panel, had shown some reluctance to forge ahead with the legislation but expressed frustration that Justice Roberts had refused to act or to appear before the Senate panel to discuss the issue.

“The Supreme Court is now on recess — at home with their families and traveling on vacation,” he said in announcing his plans for the legislation. “I wish them many sunny days, but even if the sun is shining there is still a shadow over the Supreme Court.”

Democrats said the reports about Justice Sotomayor showed the issue is a systemic one and that their legislation is not aimed solely at the conservative majority.

“It’s just another signal that the court needs a major cleanup,” Mr. Whitehouse said.

But while Republicans might acknowledge issues at the court, they also say it is a constitutionally created separate branch of government over which Congress has limited power.

“They’re a coequal branch of government and they’re accustomed to dealing with these issues on their own,” said Senator John Cornyn, Republican of Texas, a senior member of the Judiciary Committee.

“Just hypothetically, what if the Supreme Court sent over a code of ethics for the Senate?” he asked. “How do you think that would be received as a coequal branch of government?” Mr. Cornyn called the Democratic legislation “part of an effort to undermine public confidence in a court that they don’t agree with.”

In an opinion essay published last week in The Washington Post, Senator Mitch McConnell, the Kentucky Republican and minority leader, said, “Unfortunately, Democrats have moved from complaining about the Supreme Court’s reasoning to questioning its independence.” Mr. McConnell, a staunch defender of the court, dismissed the legislation, saying, “Democrats on the Senate Judiciary Committee are trying to tell a coequal branch of government how to manage its internal operations, ostensibly to clean up its ‘ethics.’”

Democrats counter that the argument that Congress can’t exert power over the court is false, considering that Congress already establishes the jurisdiction of the court, controls the number of justices that serve on it and funds its operations, among other things.

“The fact that it’s separate doesn’t mean that it is unaccountable,” said Senator Richard Blumenthal, Democrat of Connecticut and another member of the Senate panel. “Right now this Supreme Court is acting like a bunch of politicians seemingly answerable to no one. That is not what separation of powers is.”

The Judiciary Committee effort has the support of Senator Chuck Schumer, the New York Democrat and majority leader, who urged Republicans to get behind it.

“Without ethics reforms, the Supreme Court risks forever losing its legitimacy,” he said in a statement. “If Republicans truly care about the Supreme Court they should work with us to pass ethics reforms.”

But Mr. Schumer has not committed to bringing the legislation to the floor if it faces solid Republican opposition, though backers are encouraging him to do so.

“It’s not about Republicans or Democrats because the ethical breaches have concerned both,” Mr. Blumenthal said. “There is no plausible reason for a United States senator to oppose a code of ethics for the United States Supreme Court. You should put senators on the record.”