"I can assure you the bill ... has over 85 percent approval and that's with gun owners and everyone else saying if you go to a commercial transaction, gun show, or on the Internet, don't you think you ought to know who wants to buy that gun and for what purpose and what their background is? That's all we're talking about," he said on "America's Newsroom."
Manchin, who met with President Trump last week, said non-commercial transfers of firearms between family members are "protected" in the bill, and the Second Amendment would not be infringed upon by the proposed legislation.
Manchin said he was "optimistic" after the meeting, describing Trump as "very engaged" on the issue.
Calling the moment “critical,” Democrats Sen. Chuck Schumer of New York and House Speaker Nancy Pelosi of California sent Trump a letter on Sunday urging him to expand background checks and to use his influence to sway Republicans in the Senate who blocked the bill passed by the House earlier this year.
The letter mentioned earlier reports that Trump has signaled an openness to stricter background checks.
Pelosi and Schumer pointed to the mass shootings in August in Ohio and Texas that claimed 31 lives and told the president that he is “uniquely positioned” to answer the call of Americans for more gun restrictions.
Manchin and Toomey, who is from Pennsylvania, first proposed the bill in 2013, but Manchin said Trump is in a unique position to push it over the finish line.
"I said, 'Mr. President, there's not a person I know in America who believes Donald Trump will infringe on their Second Amendment rights or take their guns away or allow a registry to happen.' We're not going to do that at all, none of us. Without his support, I don't think anything moves," said Manchin.
Fox News' Edmund DeMarche contributed to this story.