EXCLUSIVE: A conservative think tank is spending more than $1 million to run ads during NFL and college football games over the Thanksgiving holiday with the hopes of keeping a new bill codifying same-sex marriage out of the end zone in the Senate next week, at least until lawmakers add new religious liberty protections to the bill.

The Heritage Foundation's campaign is being launched ahead of a final vote in the Senate on Monday to pass the Respect for Marriage Act, which would codify the right to same-sex marriage.

Heritage and other conservatives say the bill is deceptively named and would not deliver new rights to same-sex couples and that it leaves people of faith vulnerable to litigation and other forms retaliation.

"America’s religious liberty is under attack with this impending vote in the Senate," said Heritage Foundation President Kevin Roberts. "This legislation does not add one additional benefit to same-sex couples in the United States; it’s an attack that sets the stage to take rights away from people of faith. What it does accomplish is deputizing radical activists to target Americans who cannot in good faith endorse anything other than a man-woman marriage. The American people deserve all the facts."

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The group accuses Democrat lawmakers of "hurrying to cram through their far-left agenda" before the new Congress next year, when the GOP will be in the majority in the U.S. House of Representatives.

Heritage says the 50 Senate Democrats and 12 Republicans who voted to advance the bill last week are "sneaking" through a bill that would "expose religious schools and nonprofits to lawsuits" and worries the law could give the IRS a basis for stripping the tax-exempt status of dissenting religious groups.

Sen. Pat Toomey, R-Pa., has said it would allow activists to sue dissenting faith-based groups in an effort to "force them to abandon their deeply held beliefs about marriage or close their doors."

Sen. Mike Lee, R-Utah, offered an amendment to the legislation to solidify religious liberty protections in the bill, but so far Democratic leadership has refused to allow a vote on the amendment.

"Republican senators claiming the bill protects religious liberty are misleading the public," said Roger Severino, Heritage’s vice president of domestic policy. "Their refusal to require Sen. Lee's amendment is proof of their insincerity."

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Sen. Mike Lee, R-Utah (Bill Clark/CQ-Roll Call Inc. via Getty Images)

The 30-second ad will air in local broadcasts during the NFL Thanksgiving Day matchups between the New England Patriots and Minnesota Vikings, Buffalo Bills and Detroit Lions, and New York Giants and Dallas Cowboys. The ad will also be seen during coverage of four rival college football games, including Iowa Hawkeyes vs. Nebraska Cornhuskers on Friday and Indiana Hoosiers and Purdue Boilermakers on Saturday.

The campaign, which includes a $300,000 worth of digital ads by the group's political action arm, Heritage Action, totals $1.3 million, making it the largest advertising campaign by the group to date.

Last week, the Senate cleared the Respect for Marriage Act through a key procedural hurdle in a bipartisan vote of 62-37, and a handful of Republicans gave the measure enough votes to clear the filibuster.

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Sen. Susan Collins, R-Maine. (AP Photo/J. Scott Applewhite)

Senate Majority Leader Chuck Schumer, D-N.Y., said the bill is a "simple, narrowly tailored but exceedingly important" measure, "as personal as it gets."

Co-author Susan Collins, R-Maine, said the "bill recognizes the unique and extraordinary importance of marriage on an individual and societal level," and she touted the bill’s religious liberty and conscious protections.

But Lee said he voted against the bill "because the religious liberty protections were severely anemic and largely illusory."

New ad from the Heritage Foundation claims Democrat lawmakers are "hurrying to cram through their far-left agenda."

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"Religious Americans will be subject to potentially ruinous litigation, while the tax-exempt status of certain charitable organizations, educational institutions and nonprofits will be threatened," Lee said, adding that his amendment would have "shored up those vulnerabilities."

"It is a shame it wasn't included," he said.