To get around this requirement, the Senate just presumes there is a quorum of senators present and continues with its business. This procedure can be challenged if a senator "suggests the absence of a quorum," according to Senate rules.
After this happens, the presiding officer of the chamber asks the clerk to call the roll. While the clerk reads the roll, the Senate cannot conduct any business. This measure is usually invoked by a senator trying to delay debate or action on the floor. The roll call is typically stopped by unanimous consent just before the roll is finished, because a lack of quorum means that work in the chamber would come to a standstill.
In rare circumstances, the majority leader can request a "live" quorum call because he actually wants all senators present for debate. If a majority of senators are not present in the chamber, the majority leader can make a motion, which if agreed to by a plurality of senators, would direct the sergeant at arms to request the presence of absent senators. Usually this works, and a majority of senators come to the floor.
If it doesn't work, however, that's where things get dicey. The Senate can then direct the sergeant at arms to compel — or even arrest — senators to bring them to the floor.
Has Reid Ever Called For a "Live" Quorum?
Reid has used this tactic before, though not to the extent that senators have been arrested. In March 2010, he called for one to end an impasse over a one-month extension of unemployment insurance benefits.
He also called for a live quorum in July 2010 after hours of debate over a small-business bill and in September 2010 for a Democratic bill that targeted outsourcing.
And, in June 2011, a plurality of senators agreed to his motion to instruct the sergeant at arms to instruct absent senators to come back to the chamber.
Senators have been arrested through this procedure, though.
From The Washington Post on Feb. 24, 1988:
Sen. Bob Packwood (R-Ore.) was carried feet first into the Senate chamber by Capitol Police early today as Democrats ordered the arrest of absent senators in a dramatic filibuster showdown over a Democratic bill on spending in senatorial campaigns.
Packwood's arrest came after Democrats forced filibustering Republicans to hold the Senate floor in nonstop session through the night in an attempt to wear down their opposition to the bill.
Majority Leader Robert C. Byrd (D-W.Va.) said he regretted the arrest but said he had no alternative and congratulated Packwood "on the fine spirit with which he accepted the inevitable."
And from the Senate Historical Office, this case from Nov. 14, 1942 on a civil-rights bill:
[Sergeant at Arms] Jurney sent Deputy Sergeant at Arms Mark Trice to the Mayflower Hotel apartment of Tennessee Senator Kenneth McKellar, the Senate's third most senior member. In his book on Tennessee senators, Senator Bill Frist describes McKellar as an "extraordinarily shrewd man of husky dimensions with a long memory and a short fuse." When Trice called from the lobby, McKellar refused to answer his phone. The deputy then walked up to the apartment and convinced the senator's maid to let him in.
When Trice explained that McKellar was urgently needed back at the Capitol, the 73-year-old legislator agreed to accompany him. As they approached the Senate wing, McKellar suddenly realized what was up. An aide later recalled, "His face grew redder and redder. By the time the car reached the Senate entrance, McKellar shot out and barreled through the corridors to find the source of his summons."