Texan Republican Ted Cruz launched a marathon speech on the floor of the US Senate on Wednesday in a theatrical effort to draw attention to his flagging attempt to block Obama's healthcare reforms.

Though no longer able to delay proceedings significantly with a formal filibuster, Cruz began speaking out against Obamacare at 2.40pm and vowed to keep going until he was "no longer able to stand".

Hours beforehand, the first-term conservative senator was thwarted in his effort to link Obamacare to a wider federal budget resolution, significantly reducing the threat of a government shutdown next week.

Republican Senate leaders Mitch McConnell and John Cornyn refused to back a Cruz plan to block the resolution, opening the way for Democrat majority leader Harry Reid to schedule the first in a series of procedural votes starting on Wednesday afternoon.

But Cruz took to the floor nonetheless, with periodic support from fellow Republicans Mike Lee and David Vitter, to rail against Obamacare and a Senate process which he compared to a "rigged wrestling contest".

Showing signs of continuing well into the night, Cruz's pseudo-filibuster deployed the colourful rhetoric that have made him a 2016 presidential favourite among Tea Party conservatives.

"We need to make DC listen," he said. "Make them listen to the single mom working in the diner, struggling to feed her kids who has just been told she has to take a cut in hours due to Obamacare."

Invoking the Second world war, civil war, war of independence and space race in his favour, Cruz rarely skipped a beat as the afternoon wore on.

But Senate majority leader Reid was adamant that Cruz would not be allowed to frustrate passage of the bill any further. "I want to make sure everyone understands: There is no filibuster today," declared Reid at the outset of Tuesday's session.

Cruz also appeared to acknowledge that he had lost Republican support in the Senate, complaining that lawmakers on both sides were preparing to give away their filibuster rights by allowing Democrats to hold a simple majority vote on future amendments.

"Our leaders, in both parties, are asking us to vote on a bill without knowing what is in it," he said.

Reid will bring a so-called 'motion to proceed' vote on Wednesday followed by a 'cloture' vote to end debate on Friday or Saturday. This requires 60 votes to pass, but without the support of their party leadership, Cruz and Lee are unlikely to stop it going through.

After that, Reid is free to introduce an amendment on Sunday to the original budget resolution which strips out the threat to defund Obamacare and only requires 51 votes to pass.

This would mean a so-called "clean" budget resolution will be sent back to the House on Monday with only hours to go until the government funding authorisation expires.

Republicans may still choose to force a government shut down at this point, but speaker John Boehner is increasingly expected to allow a vote that would pass the budget resolution with help from Democrats and a minority of House Republicans.