By 8 a.m. EDT Wednesday, Cruz and his allies had spoken for more than 17 hours, the fifth-longest Senate speech since precise record-keeping began in 1900, The Associated Press reported.
There was no sign that the weary Cruz was prepared to stop until the Senate reconvenes later in the morning for a vote he cannot stop.
As he took the floor Tuesday afternoon, Cruz declared, "I rise today in opposition to Obamacare." But all the posturing amounts to little more than a very, very long speech. The display is not formally considered a filibuster because it is not being used to stop legislation—in this case a bill that would continue to fund the government.
(Read more: More Americans oppose defunding)
The Senate will still proceed with a vote on Wednesday to take up legislation passed last week by Republicans in the House which would prevent a government shutdown but also contains provisions to stop funding the Affordable Care Act. Cruz is protesting the bill because Senate rules would allow Democrats to strip the part that would defund Obamacare.
"I intend to speak in support of defunding ObamaCare until I am no longer able to stand, to do everything that I can to help Americans stand together," the hard-charging Texas senator said.
Because of Majority Leader Harry Reid's move Monday to schedule a test vote on government funding for Wednesday morning, Cruz can't do anything on his own procedurally to delay the timing of that vote.
And as Cruz spoke, Reid aide Adam Jentleson tweeted that the Republican "pre-negotiated the terms of his #fakefilibuster with Senator Reid yesterday. Not exactly a Mr. Smith moment."
When asked how long he planned to speak, Cruz offered a wry response to reporters on Capitol Hill: "We shall see."
If Cruz managed to keep at least 60 of his fellow senators from supporting that Wednesday vote, he could prevent cutting off debate on the budget bill – and that would be a filibuster.
Cruz wants that procedural vote to fail, because—if the final government funding bill subsequently passed, Democrats would simply strip out the part of the legislation that deals with Obamacare, kicking the clean bill back to the House.
But Cruz has only a few allies in that attempt. Some of those supporters, Republican Sens. Mike Lee, David Vitter, Pat Roberts, Jeff Sessions, Marco Rubio, Jim Inhofe, and Mike Enzi had visited the Senate floor to ask their colleague a question, a tactic that allowed Cruz a temporary break from speaking.
Kentucky Republican Sen. Rand Paul, who in March held a 13-hour filibuster, also took to the Senate floor to ask if Cruz would accept any sort of compromise that keeps the government open while revising the health care law.
Republicans "don't control all the government" Paul said while asking Cruz if he intended to shut down the government.
Cruz said he did not want to shut down the government, but would not be open to a middle ground and "will not vote for a continuing resolution that does not defund 'Obamacare.'"
Paul actually privately opposed Cruz's approach to the stopgap spending measure during a meeting with other GOP senators on Tuesday. Sources told NBC News that Paul joined with Minority Leader Mitch McConnell, R-Ky., to help hasten the return of the spending measure to the House to give colleagues in the lower chamber more time to figure out how to proceed.
But most senior Republicans, including McConnell, will not be coming to Cruz's aid. They have said they will vote to cut off debate— which would enable Democrats to strip the provision to defund Obamacare, but—more importantly—also allow the process of reaching an agreement to fund the government to move forward.
On that note, Reid said Tuesday that he would seek to move forward with legislation to fund the government through mid-November, a more modest time frame than had initially been sought by lawmakers.
The political appeal, though, of a long speech—filibuster or no—is undeniable. Paul earned the adulation of conservatives for the filibuster he waged against the Obama administration's national security practices and its use of drones.
Paul's effort back in March to hold up the president's nominee to lead the CIA was the first time the tactic had been used since 2010.
"We saw something incredible happen at that time...and it transformed the debate," Cruz said Tuesday, commending Paul's effort.
Cruz, like Paul, has possible designs on the Republican presidential nomination in 2016, and his speech against Obamacare – the program so hated by conservatives – could help endear him to the party's base.
—By Michael O'Brien, Carrie Dann and Andrew Rafferty, NBC News; Kelly O'Donnell and Matthew DeLuca contributed to this report.