As the White House and congressional leaders woo votes to authorize military intervention in Syria, certain lawmakers serve as important bellwethers — and potentially critical components — to the math of 218.

Military action in Syria is, ultimately, a policy vote. But it is also, inescapably, a political one.

Voting against Syria could forever mar a Republican as soft on defense, just as voting for it could brand a Democrat a war hawk. And how members vote could play roles in leadership elections down the line. Elections could be won or lost and legacies built or dashed based on this vote — and lawmakers know it.

There are many votes that could be insightful gauges: Kosovo, Libya, perhaps even the fiscal cliff.

But the last time the White House and Republican and Democratic leadership were on the same side of a key vote was the National Security Agency amendment from Michigan Republican Justin Amash. The NSA amendment pitted libertarian Republicans and liberal Democrats against establishment lawmakers concerned with national security. It came within seven votes of adoption.

It was also the first time a majority of Democrats sided against President Barack Obama on a national security issue. And, as one Democratic aide put it, Democrats learned that “the sky won’t fall” if they vote against Obama.

Another way to handicap the vote is by looking at individual legislators.

These 20 so-far-undecided House lawmakers are ones to watch:

Republicans

Kevin McCarthy of California

The No. 3 Republican in the House has yet to decide if he’ll join Speaker John A. Boehner, R-Ohio, and Majority Leader Eric Cantor, R-Va. He probably will, but his reluctance already indicates just how tough a vote this could be. And the last thing Boehner and Obama need is the guy in charge of counting votes not on the team.

Paul D. Ryan of Wisconsin

Ryan, the proverbial GOP thought-leader, carries a great deal of weight in the House. Like McCarthy, he will probably side with leadership. But this vote also will be one of the defining ones heading into the 2016 presidential race, should he make a run.

Cathy McMorris Rodgers of Washington

The No. 4 Republican still hasn’t said which way she is leaning, but if she sided against Boehner and Cantor, it would be significant.

Her staff says she is “carefully considering” intervention and she is “awaiting final legislative text,” so she is unlikely to announce her position soon. McMorris Rodgers will likely have a strong sense of how Republicans are leaning when she makes up her mind.

Tom Price of Georgia

Price says he is still undecided, but whichever side he comes down on could enjoy a big boost. Price is well-respected in the Republican ranks, and many Southern Republicans look to him for guidance.

Bill Huizenga of Michigan

Huizenga is a middle-of-the-road Republican who doesn’t like to make trouble. But he has just enough of an independent streak to break from Republican leadership. On the Amash NSA amendment, Huizenga was undecided up to the last moment. He voted for the amendment, but multiple sources say he was willing to flip if leadership needed him.

Cory Gardner of Colorado

The sophomore Republican has made a name for himself as a conservative to watch. Gardner is likely torn on the issue of Syria. He wants to please leadership, and he wants to please his young libertarian-leaning friends. He doesn’t want to be soft on defense, but he doesn’t want to start a war. His dilemma is a lot like the dilemma facing other Republicans.

Jim Jordan of Ohio

When conservatives want to know where conservatives stand on an issue, they often look to Jordan — the former chairman of the Republican Study Committee.

Scott Rigell of Virginia

Rigell’s prominence rose when he led the charge to get President Barack Obama to go through Congress for authorization of Syria. Now that Obama has gone that route, Rigell seems inclined to vote no, but if he were to back Obama, that could prove significant.

Ann Wagner of Missouri

Wagner is a freshman to watch in general. The ambitious Republican is eager to please leadership, but she is also trying to maintain her conservative credentials among her young colleagues. She is one member constantly in communication with her colleagues. If the GOP mood swings against Syria, Wagner will pick up on it.

Tom Cole of Oklahoma

Cole — a close Boehner ally — seems to be leaning against intervention in Syria but could still go the other way. Cole is suspicious of any bombing campaign that doesn’t end with the United States securing chemical weapons.

Democrats

James E. Clyburn of South Carolina

The No. 3 Democrat in the House is not afraid to vote against leadership and the White House. He did just that when he recently voted for the Amash NSA amendment. But the former Democratic whip and former Congressional Black Caucus chairman has in the past been inclined to support the president and could prove key in getting Obama’s base behind him — or not.

Marcia L. Fudge of Ohio

The leader of the CBC is a key vote. She, Clyburn and Emanuel Cleaver II, D-Mo., a former and unofficial leader of the CBC, could help sway the outcome. They face a substantial dilemma: oppose the war or oppose the president.

Xavier Becerra of California

The House Democratic Caucus chairman told CQ Roll Call over the weekend that this was a vote of conscience. So if Becerra’s conscience is leaning one way or another, that could be a good indication of the overall direction of the Democratic conscience. Becerra will probably support intervention, but if he doesn’t, it would be a bad sign for Democratic leadership.

Joseph Crowley of New York

The vice chairman of the Democratic Caucus has yet to state where he stands, but if he comes out against striking Syria, many Democrats will notice. Crowley defied the White House to back the Amash NSA amendment.

Rosa DeLauro of Connecticut

DeLauro is a close Pelosi ally. She’s also a liberal conscience who isn’t afraid to break from leadership.

Jan Schakowsky of Illinois

Schakowsky is another one of Pelosi’s best friends — and she, too, is undecided. Now in her eighth term, she’s been in the House long enough to have seen the consequences of authorizing military force. Schakowsky is also on the Intelligence Committee. If Schakowsky votes no, other members might see it as a sign that the intelligence on Syria is not compelling.

Jim Himes of Connecticut

Another member of the Intelligence Committee, Himes can speak forcefully and convincingly on Syria. He seems likely to vote against intervention, but if he comes out for striking Syria, Democrats will listen.

Himes, like many Democrats, faces his own dilemma. He came to Congress in 2008 after a war-weary public ushered in a Democratic majority. But Himes is also a fan of Obama. He said recently it “pains” him to oppose the president on such a key issue as Syria. If Himes can do it, other Democrats could follow.

Jared Polis of Colorado

Polis seems open to the idea of a limited strike “if we have the ability to do it, without adversely affecting our security.” But Polis is a member whose position will likely depend on the details of the resolution. If he is convinced by an argument that, for example, a resolution is the best way to restrain the president’s use of force, Polis could be an important ally.

John B. Larson of Connecticut

The former chairman of the House Democratic Caucus seems open to backing a strike as long as it doesn’t mean “boots on the ground.” Once again, the text of the resolution and the mood of the caucus will matter. Whichever way he is leaning is a good indication where other Democrats are headed, too.

Zoe Lofgren of California

Lofgren calls herself a “skeptic,” but she says she will give the question “the kind of thorough consideration that such a question deserves.” Lofgren is known to concern herself with the details, and she was elected to Congress during 1994′s Republican wave. If she signs off, other Democrats may trust her judgment.

Emma Dumain contributed to this report.