Senate Majority Leader John Thune (R-S.D.) is tossing a curveball into the shutdown fight as he plans to bring a full-year spending bill for the Pentagon to the floor on Thursday, effectively daring Democrats to oppose it as part of their push to keep health care at the forefront of the impasse.

Democrats have been almost completely united in their opposition to the GOP's "clean" bill to fund the government through Nov. 21. The Senate voted against the House-passed continuing resolution for the ninth time on Wednesday, with another vote on the stopgap expected on Thursday.

But Thursday's vote on the Defense bill represents more of a gamble, and leaders on Wednesday largely said they hadn't yet decided what to do.

The full-year bill is the result of a bipartisan appropriations process and passed out of committee on a 26-3 vote earlier this year. And it would fund military paychecks that could be on the line again if the shutdown drags until the end of the month.

At the same time, Democrats want to appear steadfast in their opposition to the Trump administration, and note that they've gotten no guarantees about what Republicans could attach to the Defense bill once it clears Thursday's procedural hurdle.

"It seems like it's a hard vote," Thune told The Hill on Wednesday. "Because they all say they want a normal appropriations process, and we're trying to give them one. I get it, it's in the middle of a shutdown, which is a complicating dynamic here."

"We need to get the appropriations process going either way," Thune said. "If we're sitting around here voting every day and they keep voting to keep the government shut down, we need to be trying to move the needle on some of the other stuff that we need to get done."

On top of the Pentagon spending bill, the Senate is also set to consider a measure to allow lawmakers to conference with the House on a three-bill "minibus" that was green-lighted earlier in the year. Thune and Senate Republicans are also eyeing attaching a number of other spending bills to the Pentagon legislation.

Among those is the Labor-Health and Human Services bill, which would be done in a bid to win over Democrats. Senate Appropriations Committee Chair Susan Collins (R-Maine) also told reporters that she is hoping to attach the Transportation and Housing and Urban Development (THUD) bill and a separate one for the Departments of Commerce, Justice and science agencies (CJS).

But whether Democrats will play ball is a major question. Multiple key party leaders demurred when pressed on Wednesday over how they will handle the vote.

Senate Minority Leader Chuck Schumer (D-N.Y.) on Wednesday said Democrats needed to see what Republicans would try to attach to the bill before making up their minds.

"We do not know what they're going to offer yet," Sen. Patty Murray (Wash.), the lead Democratic appropriator, concurred. "Let's see what they do tomorrow."

The Senate will technically vote on the House-passed Defense appropriations bill, but only as a vehicle. That language is widely expected to be swapped out for the Senate's version along with the other funding bills the two sides agree to.

If the bill clears the procedural hurdle, the chamber would then move toward a substitute item -- but it would need consent from the entire chamber.

That too remains a jump ball, especially as Democrats continue to cry foul over the possibility of the Trump administration taking more steps to claw back on a partisan basis funds that had passed the Senate with bipartisan support.

"But as best I can tell, Patty Murray has been shown no hint of bipartisan cooperation or any willingness to put any guardrails around what they plan to do," said Sen. Sheldon Whitehouse (D-R.I.). "I think the stage we're at is that we've been showed nothing, so there's no reason to vote for it yet."

"What was needed is a larger agreement about how the appropriations process is going to move forward, so it's clear that our priorities are respected," he added.

Sen. Jeanne Shaheen (D-N.H.), an appropriator, has been a supporter recently of moving the full-year items, especially as they contain Democratic wins. Nevertheless, she is sticking with the party line as they await a final word on what will get attached.

"I think it's helpful. My understanding is that there are going to be other bills put on it, and then if we have a commitment to move forward with appropriations, it helps us," she said, declining to say whether she is an "aye" or "no" vote.

A failed tally on the Defense bill would also open Democrats up to attacks of blocking future payments for military members, which are due again in two weeks after the Trump administration announced plans to pay them on Wednesday with $8 billion in unused research and development funds.

In the House, Speaker Mike Johnson (R-La.) indicated Wednesday he would not bring the lower chamber back to vote on a full-year Pentagon bill until the government reopened, while also voicing doubts that the measure would pass the Senate anyway.

The Senate bill contains some marked differences from the House version, which the lower chamber would need to approve. Johnson has kept the House out of Washington, D.C., for the duration of the shutdown in at attempt to jam Senate Democrats.

"My suspicion is the Democrats are going to play their same political games and stop that cold," Johnson said. "I hope I'm surprised by that."

Republicans in the Senate, meanwhile, are adamant that Democrats should back the measure. They were quick to note on Wednesday that the bill advanced in late July by an overwhelming majority, and that the current state of play should change little with how they consider the funding item now.

"This is a bill that passed the committee ... 26 to 3," Senate Majority Whip John Barrasso (R-Wyo.) told The Hill. "You don't think you can bring a bill out of the committee that passed 26 to 3 and the Democrats are going to change their mind?"

Senate GOP, Thune throw curveball into shutdown fight