Senator Feingold of Wisconsin is offering the most detailed legislative proposal yet to rein in the essentially unfettered authority customs officials have claimed to search laptops, cell phones, and other electronic devices travelers bring into or out of America.

Under a 29-page bill Mr. Feingold introduced on Friday, customs agents at airports and borders would need to document a “reasonable suspicion” before inspecting a computer or similar device carried by an American resident and could only hold on to the device for 24 hours before starting the process of seeking a warrant from a judge.

“Requiring citizens and other legal residents of the United States to submit to a government review and analysis of thousands of pages of their most personal information without any suspicion of wrongdoing is incompatible with the values of liberty and personal freedom on which the United States was founded,” a preamble to the bill declares.

Officials at the Department of Homeland Security have asserted that customs agents need no particular suspicion to open, inspect, and copy laptops just like any other property carried across a border. At least two appeals courts have endorsed the government’s view.

A spokeswoman for the Department of Homeland Security, Amy Kudwa, said a department policy prevented her from commenting on pending legislation. However, in a letter to Mr. Feingold earlier this month, an assistant secretary of homeland security, Donald Kent Jr., warned about encumbering customs agents.

“Requiring officers to specify individualized suspicions would drastically limit their ability to gather travelers’ information effectively,” Mr. Kent wrote.

In recent months, Congress has heard complaints that laptop searches were becoming more frequent and intrusive. Travelers from Pakistan and Middle Eastern countries have complained about the frequency of such searches. Some business groups and law firms have also expressed concern that their confidential information could be exposed.

Between August 1 and August 13 of this year, 40 of about 17 million people entering America had their laptops inspected by customs officials, according to Mr. Kent. He called the search policy “a minimal burden on travelers.”

Mr. Feingold’s legislation would require that a supervisor approve and be present for every laptop search. Travelers would also have the right to be present during such a search, which would have to be conducted in an area away from public view. The measure also contains a prohibition on profiling by “race, ethnicity, national origin or religion.” However, the protections in the bill do not apply to foreign visitors to America.

A homeland security expert at the Heritage Institution, James Carafano, said the legislation has no chance of passage because it could be blamed for a future terrorist attack. “No Congress is ever going to put its name on this bill because if they do after the next heinous act the sponsors of the bill will go down in infamy,” he said.

Mr. Carafano said there is no reason to give more protection to information on electronic devices than to any papers, photographs, or other files people bring across the border. “You’d just be saying to people you can walk into this country with a sanctuary on your back,” he said.

“Requiring any degree of suspicion at the border means DHS’s ability to catch things like child pornography or materials from terrorist cells will be diminished,” a former prosecutor who is now a professor at St. John’s University in Queens, Larry Cunningham, said. “I like the provisions about supervisor approval and not doing this in public, but I’m concerned that the reasonable suspicion standard is setting up a category of ‘closed containers’ that will essentially be immune from search.”

Senators Akaka of Hawaii, Cantwell of Washington, and Wyden of Oregon, all of whom are Democrats, are co-sponsoring the legislation. Two House Democrats, Rep. Zoe Lofgren of California and Rep. Eliot Engel of New York, recently introduced shorter bills seeking to impose a “reasonable suspicion” standard on border laptop searches. Because of the November election and high priority legislation, it is doubtful that any of the measures will pass this year.