Democratic lawmakers are starting to get behind a Trump administration funding proposal for the national park system after expressing significant doubts about it.

The plan, which would use revenue from energy sales on federal lands and offshore to plug the maintenance backlog for the national park system, now enjoys bipartisan support just a few months before the midterm elections. Proponents of the measure are optimistic it can pass in this Congress.

Three House Democrats joined Republicans Wednesday to introduce a bill, dubbed the Restore Our Parks and Public Lands Act, that endorses using the income from oil and gas drilling on public lands to fund construction projects for the National Park Service (NPS), Bureau of Land Management, Fish and Wildlife Service and Bureau of Indian Education.

In total 25 Democrats and 25 Republicans signed onto the bill.

They join six Democrats and 10 Republicans in the Senate who are backing legislation to dedicate a portion of energy money to national parks infrastructure. Interior Secretary Ryan Zinke started floating a similar idea last year, in part to promote the administration's desire to expand oil and gas lease sales on federal land and offshore.

The bipartisan Senate bill introduced in June is co-sponsored by Sens. Mark Warner (D-Va.), Rob Portman (R-Ohio), Lamar Alexander (R-Tenn.) and Angus King (I-Maine).

For at least some of the Democrats, signing on to the bills represents a reversal from their previous skepticism of Trump's plan.

Rep. Raúl Grijalva (D-Ariz.), a leading progressive and ranking member of the House Natural Resources Committee, previously denounced Zinke's proposal for relying on "funny money."

"It's funny money," he said in February. "[Interior is] betting on the idea that opening up the public lands will generate revenue. Stupidly, mineral extraction doesn't provide one penny of royalties to anybody."

At the time, Grijalva added, "Gas and oil royalties pay 8, 9, 10 percent depending, and that is not going to be enough. There are already thousands and thousands of permits that have never been used."

Part of Zinke's proposal, not reflected in the House bill, included using only funds generated from increased oil and gas drilling on public lands and waters. The House bill proposes to use funds from current energy production. Democrats say that the new proposal is an improvement over what Zinke wanted.

Grijalva is a co-sponsor of the House bill, along with House Natural Resources Committee Chairman Rob Bishop (R-Utah), whom he frequently spars with on legislation.

Bishop said while he's not opposed to including the bill in larger must-pass legislation, he believes the bill has enough political clout to pass as a stand-alone and said he's working with senators who introduced similar legislation. The senators said it's unlikely the bill would move before the August recess.

While Grijalva championed the bill during a press conference Wednesday morning as an "investment in our parks," he later acknowledged that the legislation isn't a perfect fix for the roughly $12 billion maintenance backlog.

"It's a hell of a start on the $12 billion, but it's not the end-all be-all," Grijalva told The Hill.

"It's a way to provide funding for the stuff that needs to be fixed. Long term? No, but what's good is it has more money to give permanently to [parks]. We can't keep pulling rabbits out of our hat every five years," he said, referencing the Land and Water Conservation Fund (LWCF), another Interior program up for renewal that funds conservation at public parks.

Under the House and Senate proposals, half of the money taken in by the federal government annually from energy production on federal land and offshore and not dedicated for another use would go into a fund. The House wants that fund available for numerous agencies, while the Senate's bill would preserve it just for the NPS.

Zinke endorsed the Senate legislation and Bishop said he supported the House's version. Interior didn't respond to a request for comment on the House bill.

Other Democrats who back the bill from Bishop and Grijalva acknowledged concerns that the concept could promote more drilling or that there isn't enough money generated from energy revenues that hasn't already been spoken for.

But sponsors say they went to great lengths to address those concerns, including removing parts of the bill that could be seen as incentivizing additional drilling on public lands - a hot topic of concern from environmentalists who worry the administration will expand fuel extraction across the U.S.

A number of environmental groups back the House bill, including the National Parks Conservation Association, which was previously skeptical of Zinke's plan.

"I think the bill was put forward in a responsible way and in a way that solves a real problem," said Rep. Derek Kilmer (D-Wash.), who rolled out the legislation Wednesday with the bipartisan group.

Rep. Kurt Schrader (Ore.), a moderate Democrat, said Grijalva's participation in the process shows that the left shouldn't worry about the bill.

"There was some concern, I think fairly legitimate, that the way the original bill was written there would have to be increased production, potentially, of our energy resources for money to flow into this parks bill. That's no longer the case," he said.

"Getting the ranking member on was a big deal. That was probably his major concern, that it's going to push us to do uncontrolled oil and gas exploration on public lands. And that's not the case under the new bill."