Republican Senate primary challengers are looking to use Syria as a wedge to gain traction in their bids to unseat GOP incumbents. [WATCH VIDEO]

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And nearly every senator facing a contested primary — aside from Graham, who has expressed support — is still undecided, a week after President Obama said he’d go to Congress for approval.

The vote is one that could provide some much-needed traction for conservative challengers who have, thus far, largely been unable to spark the kind of grassroots furor that fueled incumbent turnover during the 2010 elections.

And the silence from incumbents, warns Larry Sabato, director of the University of Virginia’s Center for Politics, could be the most damning. He warned that McConnell might have the most to lose if he hesitates in expressing an opinion one way or the other.

“The only thing worse than voting ‘yes’ or ‘no’ is voting ‘maybe,’” he said. “If you say you’re a leader and your incumbency matters, you have to demonstrate it in national issues like this one.”

“McConnell could find that out if he delays a decision till the very end.”

Matt Bevin, challenging McConnell for the GOP nomination, has accused McConnell of a lack of leadership on the issue — an attack that cuts at the core of McConnell’s pitch to Kentucky voters, that his seniority and influence in the Senate benefits Kentuckians.

“I’m offended, frankly, by the non-stance of Mitch McConnell,” Bevin told Breitbart.com.

McConnell’s Democratic opponent, too, has criticized him for his silence.

In Tennessee, Joe Carr, a conservative state representative challenging Alexander in the primary, echoed that charge in an interview with The Hill.

“All we hear from Sen. Alexander on this issue is crickets. I think the state of Tennessee is demanding, and will get, someone who is going to show leadership,” he said.

For Enzi, the Syria vote and his early indecision, could become a “defining issue” in his primary challenge, Sabato warned.

“He has all the chips in his corner among Wyoming Republicans, but he's handing this issue to Liz Cheney,” he said.

Cheney has struggled to draw a contrast between her and the incumbent, who has a 92 percent rating from the American Conservative Union. And she’s faced negative press in recent weeks surrounding a report that she offered false information to obtain a fishing license.

The Syria vote allows her to pivot back to issues that will excite the base and gives her the rare opportunity to come out on top of her already considerably conservative opponent on an issue.

The attacks on the incumbents’ silence also give the issue legs far past the actual vote and its outcome. Carr said he plans to continue to keep the focus on Syria because it’s indicative of a wider trend for Alexander of failing to lead — a charge that has no expiration date and one he can maintain even after Alexander eventually takes a side.

It’s very possible that these undecided senators will all vote against military engagement.

But the overall silence reflects the fact that the issue has caught fire with the conservative wing of the party, and if the senators come out on the wrong side of the vote, they could face significant backlash at the polls on primary day.

The problem for these incumbents is twofold: A vote to approve the use of military force is a vote both for a policy proposed by President Obama, and a vote in favor of a conflict heavily disapproved by Americans.

Already, conservative groups are sounding the alarm on military engagement in Syria. FreedomWorks announced its opposition to the conflict on Friday. Before that, TheTeaParty.net and the American Conservative Union expressed opposition, and conservative group Citizens for the Republic said it would count the Syria vote double in its annual scorecard.

The vote is likely to be more trouble for some incumbents than others.

McConnell has long had difficulty with the conservative base, but he’s worked to shore up that support, gaining the endorsement of his Tea Party-backed colleague, Sen. Rand Paul Randal (Rand) Howard PaulSecond GOP senator to quarantine after exposure to coronavirus GOP senator to quarantine after coronavirus exposure The Hill's Morning Report - Sponsored by National Industries for the Blind - Trump seeks to flip 'Rage' narrative; Dems block COVID-19 bill MORE (R-Ky.).

But Paul has been a leader of the opposition to Syria strikes, and McConnell’s left with the unenviable position of either risking some of the conservative credibility he gained with Paul’s endorsement by voting against him, or becoming the only Republican leader to oppose the strikes, jeopardizing his leadership chops.

In the meantime, Graham’s willingness to express support for engagement is reflective of the fact he’s safer than the others. His opponents are weak, observers say, and Graham, as a national leader on foreign policy, will be given the benefit of the doubt on the issue.

But opponent Lee Bright, a South Carolina state senator, said Graham’s position in favor of military engagement could undermine his strongest advantage in the race.

“The big perceived advantage for Graham has always been his foreign policy experience, and now he is making some mistakes and following Obama's zeal for the Muslim Brotherhood’s growth in the Middle East,” he said.

With the outcome of the vote uncertain, President Obama has a national address planned for next Tuesday that could shift the public dialogue.

But conservative opposition to military engagement is unlikely to wane — and these incumbents are unlikely to be able to find a way out of what looks to be one of the toughest votes of the cycle.