WASHINGTON — U.S. Sen. Cory Gardner is staying mum until the bitter end.

Hours before the U.S. Senate was set to hold a critical vote Tuesday on health care, Gardner aides said the Colorado Republican still hasn’t decided whether he would back it — an approach that tracks with Gardner’s weeks-long avoidance of a definitive position.

The ambiguity is in line with the vote itself, a procedural motion on a health care bill whose aim was unknown to most Americans — let alone lawmakers — the night before it was scheduled to take place. While the vote isn’t for final passage, and only would allow the Senate to proceed to debate on the legislation, the results are critical to GOP efforts to pass health care legislation.

On what exactly was still unknown hours beforehand.

If the procedural vote is successful, Senate Republicans could move forward on a few options, including a straight repeal of the Affordable Care Act, some version of the health care bill that Republicans have debated for weeks or a new plan to undo parts of the ACA, also known as Obamacare.

The uncertain path is the latest step in a Senate bill-writing process that has unfolded with little transparency and without a single hearing in the upper chamber.

U.S. Sen. Michael Bennet, D-Colorado, blasted that approach Tuesday morning.

“Today we are voting to consider a #healthcare bill w/o knowing what it will do or how it will affect CO,” he wrote on Twitter.

Later, in a message to Senate Majority Leader Mitch McConnell he added: “the American people deserve to know what we are voting for.”

Gardner previously has criticized the lack of public hearings, but it’s unclear what he has done or said behind closed doors to compel his colleagues to hold them. As chair of the National Republican Senatorial Committee, he is a member of Senate GOP leadership.

While he’s said little about his intentions, Colorado liberals and conservatives have said that they expect Gardner to vote in support of Republican efforts to unwind the Affordable Care Act. The first-term senator also is not considered a “swing vote” by national prognosticators.

“While he has called for changes to the process – any vote he takes isn’t on process but is on if the measure is good or bad for Colorado,” said Casey Contres, a Gardner spokesman, in a statement.