(CNN) A bipartisan group of senators said Wednesday that they believe their bill to raise the age to buy tobacco and e-cigarettes can pass Congress and be signed into law -- but only if they can keep it free of loopholes and fend off efforts to water it down or load it up with controversial provisions.

It's a challenge, they warned, made even greater by the complex and shifting legislative battlefield over tobacco.

Sen. Brian Schatz, a Hawaii Democrat, said major anti-smoking organizations, like the American Lung Association, are backing their bill for one major simple reason: "It has no loopholes, it has no exceptions, there are no tricks. It is a clean piece of legislation."

The Tobacco to 21 Act -- which aims only to raise the purchasing age from 18 to 21 -- is sponsored by Schatz, Democratic Sen. Dick Durbin of Illinois and Republican Sens. Todd Young of Indiana and Mitt Romney of Utah.

It is also backed by the tobacco industry, which has heavily promoted its support for raising the age limit, including in a full-page ad in The Washington Post on Wednesday.

Schatz acknowledged that he is skeptical of the industry's support.

"As long as we have our lawyers scouring the language for loopholes or problems, and as long as we hold the pen, I'm confident this is a good public health measure, despite the support of the tobacco industry," he said.

Schatz said he didn't know exactly how opponents of the legislation might try to change it but that they would be on guard.

"We don't know if there will be an attempt to water it down," he said. "I think we ought to expect it, but we don't know what tactically they will try to do."

After a Capitol news conference to promote the bill, Schatz explained that when they drafted the legislation they were looking for the surest way to get it enacted, and that meant leaving lots of other controversial anti-tobacco proposals -- such as banning kid-friendly flavors for e-cigarettes -- out of the legislation.

"It's as clean as clean can be, which is not to say it's a comprehensive tobacco strategy. It's just the most important one thing that can be done quickly," he said.

The Kentucky Republican, who is up for reelection in 2020, has announced that he will soon introduce his own legislation to raise the buying age to 21. While the details of his bill haven't been released, he did say it would have an exemption for members of the military who are under 21, something Schatz said was an unacceptable loophole.

Could a standoff over that seemingly minor provision kill the entire effort to raise the age by splitting the Senate so the bill can't get the 60 votes it needs to break a filibuster? Perhaps.

McConnell's role is key because he controls the floor and can decide which legislation will get there. He comes from a state that has a rich tobacco history but has suffered economically as tobacco has become less popular because of its health risks.

Steve Callahan, a spokesman for the tobacco giant Altria, told CNN his company supports both bills.

"We support these clean bills that will raise of age of purchase to 21," Callahan said, although he said his company has not weighed in on the military exception.

At the news conference, Matthew Myers, who heads the Campaign for Tobacco-Free Kids, urged lawmakers to pass "this strong legislation and to reject any efforts whatsoever by special interests to either water down this law or add provisions to this law that would provide more protection for the tobacco companies than for our nation's youths."

Anti-smoking activists worry that any changes could prevent a bill from getting 60 votes and that some of those changes might be disguised as positive proposals but actually be designed to keep the bill from passing.

"Sometimes what happens is a good piece of legislation has people who add things to it that also sound really good -- where there is a lot of support for it -- but they know that there is not enough support for it to get to 60 votes," Romney said. "So they kill something by making it sound better, even knowing it can't get across the line. "

In explaining his support for the bill, Young lamented that Indiana ranks 45th in the country for tobacco use and said revising the law is the quickest way to change that.

"The most achievable, the most consequential and the most impactful step we can take at the federal level to bring down health care costs and improve health outcomes is to pass this bill. To increase the tobacco purchase to 21," Young said at the news conference.

Durbin made clear he wants to ban vaping flavors -- such as "fruit medley, gummy worms, cotton candy" -- that he said are aimed at young users but he said that fight would come separately.

None of the senators knew firsthand whether President Donald Trump would sign their bill. But Young said he was confident that their legislation would get at least 60 votes in the Senate, pass the House and go to Trump's desk "as drafted" -- without any changes.