Senate Democrats are forcing centrist and vulnerable Republicans to take a tricky vote Wednesday aimed at forcing them to pick a side on the Trump administration's handling of Obamacare.

The vote is on a discharge petition by Sen. Tammy Baldwin, D-Wis., to block health insurance companies from selling short-term plans, a type of coverage extended under President Trump as an alternative to Obamacare plans that Democrats and the healthcare law's allies deride as “junk insurance.” The legislative maneuver is part of Democrats’ ongoing strategy to keep healthcare at the center of the conversation ahead of the Nov. 6 midterm elections.

"People say they’re for protecting people with pre-existing conditions, people say they’re for making sure people have basic standards, they have an opportunity to prove it tomorrow,” said Sen. Brian Schatz, D-Hawaii.

The resolution is a long shot in the Senate, where it has the support of 49 Democratic senators and would need to pick up two Republican votes. But it would force all Senate Republicans, once again, to take a tough Obamacare vote. Centrist Sen. Susan Collins, R-Maine, told reporters Tuesday that she was undecided about how she would vote. Sen. Lisa Murkowski, R-Alaska, who opposed earlier GOP efforts to replace Obamacare, said she would be voting against it.

The office of Sen. Dean Heller, R-Nev., one of the most vulnerable Republican incumbents, did not respond to multiple inquiries about how he planned to vote.

Baldwin is up for re-election in her state, and said in a statement Tuesday that lawmakers should demonstrate their commitment for protecting people with pre-existing conditions through supporting her measure. The mechanism she is using is a Congressional Review Act resolution, which allows Congress to cancel administration rules with a simple majority vote.

Speaking on the Senate floor Tuesday, Senate Minority Whip Dick Durbin, D-Ill., said the vote would serve to see whether Republicans would “come over and vote not only against the Trump plan, but vote for the millions of families … who are counting on quality health insurance to be there when they need it.”

Should it advance past the Senate, it is expected to be blocked in the House, and Trump has already said he would veto it. The White House said in a statement of administration policy that it would “undermine the administration’s recent efforts to expand affordable insurance options for American families, including those harmed by Obamacare’s failing insurance markets.”

Under the Trump administration’s rules, which went into effect last week, people are allowed to buy short-term health insurance plans for up to a year, and renew them twice for up to two years. The plans were allowed to be sold until up to a year until former President Barack Obama’s final year in office, but were shortened in order to try to compel more people to join the Obamacare exchanges instead.

Republicans have said the short-term offerings, whose premiums are less expensive for some, give people who don’t qualify for financial aid under Obamacare an option, arguing that otherwise they would go uninsured because they are priced out of the market. For instance, a Murkowski spokeswoman explained that Alaska's healthcare costs are still the highest in the country, and while the senator doesn't consider short-term plans to be ideal, she wants to ensure Alaskans have options and alternatives for less-expensive coverage.

Democrats argue that the plans are “junk insurance" on the basis that they do not contain the same rules as Obamacare requiring health insurers to cover people with pre-existing conditions, nor do they necessarily include coverage for medical care such as maternity services, prescription drugs, or mental health.

For this reason, patient groups representing those with chronic illnesses have sued the Trump administration to block the rules. Certain states, including California, won’t allow the sale of short-term plans.

To defend Obamacare, Democrats have sought to highlight its protections that prevent health insurance companies from turning away sick people or charging them more. Though Republicans have insisted they believe in keeping these protections, their replacement healthcare plans have received criticism by Democrats for being more limited than Obamacare.

“Ever since taking control of Congress and the presidency, Republicans have deliberately, relentlessly undermined Americans’ healthcare,” Senate Minority Leader Chuck Schumer, D-N.Y., said on the Senate floor Tuesday. “This particular policy is part of a long campaign to sabotage our healthcare system.”

The pro-Obamacare group Protect Our Care said that the vote on Wednesday gave Republicans an opportunity to prove whether they support protections for people with pre-existing conditions.

“If the GOP truly cared about protecting Americans with pre-existing conditions, they would join their Democratic colleagues on this resolution in taking concrete action to preserve the protections and essential health benefits that tens of millions of Americans depend on,” said Brad Woodhouse, the executive director for the group.

As Democrats have questioned the GOP's healthcare promises, some Republican lawmakers have mobilized to demonstrate their commitment to sick people. For instance, 10 Republicans in the Senate introduced a bill to keep pre-existing condition protections in place if a lawsuit by GOP states and the Trump administration to gut them should be successful. Three of them, Sens. John Barrasso of Wyoming, Roger Wicker of Mississippi, and Heller, are up for re-election.

Heller is the only Republican facing a difficult re-election battle, though in a poll published Tuesday he edged out ahead of his opponent, Rep. Jacky Rosen, D-Nev.

Americans for Prosperity, an organization affiliated with the libertarian Koch brothers, urged senators to vote against the resolution and said the “harmful, senseless” measure would “take away new opportunities for millions of Americans to access quality health insurance that works for them.”

“The choice is clear — either Senators want their constituents to have affordable health insurance options or they want to trap them in one-size-fits-all plans they can’t afford,” Americans for Prosperity policy manager David Barnes said in a statement.

Laura Barrón-López contributed.