Story highlights At issue appears to be whether to include a trigger to increase taxes in a GOP plan

Aides say the trigger idea wouldn't be permitted under Senate rules

(CNN) The Senate Republicans' tax bill hangs in the balance, after Senate Majority Leader Mitch McConnell announced Thursday that votes would continue into Friday and GOP leaders grapple with how they can satisfy the conference's deficit hawks.

GOP leaders had previously hoped to vote the bill out of the chamber by Thursday, but during floor debate one key proposal hit a snag.

Since the beginning, Sen. Bob Corker, a Republican from Tennessee, has staked out ground that he would not vote for a tax bill that increased the deficit by a single penny. In recent days, he hasn't backed off that pledge.

According to GOP aides and members, the Senate parliamentarian -- the official rule keeper of the US Senate -- ruled that Corker's trigger idea wouldn't be permitted under Senate rules as it was written, injecting uncertainty into a tax bill that is supposed to be the Republican Party's major legislative achievement this year and was is expected to receive a final vote by the end of the week.

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