WASHINGTON — As the Senate moves closer to a Sunday night showdown over whether it will let Patriot Act surveillance powers expire on Monday, supporters and critics of how the government has used those authorities have been using increasingly alarming language.

But there is little evidence in the history of the expiring laws — including the one that the government uses to justify the once-secret National Security Agency program that vacuums up Americans’ phone records in bulk — to support the arguments that either side is making.

Republican senators who want to keep the program are warning that any lapse in “this critical tool would lead to attacks on the United States,” as Senator Tom Cotton, Republican of Arkansas, recently put it. Yet throughout the program’s existence, it has never thwarted a terrorist attack, studies and testimony show.

At the same time, proponents of ending the program say it poses risks to Americans’ private lives, by permitting the government to know who has been calling psychiatrists or political groups, for example. But despite the discovery of technical violations of the rules several years ago, no evidence has emerged that the program has been misused for political or personal gain. As a result, the privacy-minded critics have had to couch their warnings in hypothetical terms.