Washington (CNN) The House Freedom Caucus, the vocal group of conservatives and a constant thorn in the side of Republican leaders for years, has led the charge against the GOP bill to repeal and replace much of Obamacare.

They claim to have the votes needed to block the bill in the dramatic vote Thursday.

But if they can't deliver, it may be a body blow that forever damages the group's reach and prominence as a credible force in the House's right flank. And it could cement President Donald Trump's dominion over the conservative voters Freedom Caucus members represent.

Trump is a formidable new foe for the Freedom Caucus. No longer is it simply them vs. House leaders with bills that have no chance of being signed by former President Barack Obama. This time, the group is up against their own Republican President, who with all his policy shortcomings, is widely viewed as the ultimate closer.

Tuesday morning, Trump arrived on Capitol Hill ready to apply the pressure to Republicans in a closed-door meeting. The President called out Freedom Caucus Chairman Mark Meadows by name, warning members in the meeting that they could lose their seats in the 2018 election or in primaries if House Republicans couldn't deliver on their seven-year promise to repeal Obama's Affordable Care Act.

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