By David Morgan

CANONSBURG, Pa. (Reuters) - Republicans are scrambling to avoid a political embarrassment in a conservative district of Pennsylvania, where a pro-gun, pro-union Democrat who opposes abortion could be about to win a congressional election in one of President Donald Trump's white, working-class strongholds.

Democrat Conor Lamb, a 33-year-old Marine veteran and former federal prosecutor, is in a dead heat with Republican state Representative Rick Saccone in Tuesday's special election for an open seat in the U.S. House of Representatives that Republicans have held since 2003.

The race is seen as a referendum on Trump and a harbinger for November's congressional midterm elections, according to pollsters and party insiders who say the moderate Democrat could emerge as a model for Democratic candidates in other competitive House districts that Trump carried in 2016.

"The Democrats have nominated the ideal candidate, because he fits the profile of the electorate. If they'd nominated a liberal who allowed the campaign to become nationalized, it would be over by now," said G. Terry Madonna, director of the Franklin and Marshall College Poll in Lancaster, Pennsylvania.

"If he wins, or even if he just comes close, he is a model for Democrats in competitive Trump districts nationally."

Trump won the district, in southwest Pennsylvania, by nearly 20 points in 2016 and has endorsed Saccone.

"The Economy is raging, at an all time high, and is set to get even better. Jobs and wages up. Vote for Rick Saccone and keep it going!" the president tweeted on Tuesday.

Still, Lamb has fought to make the contest a local referendum and said he was focused on issues in his district even as he welcomed Democratic enthusiasm over the race.

"This is a local race," Lamb told reporters after voting at a church in his hometown of Mt Lebanon shortly after sunrise on Tuesday. "I don't think it has anything to do with the president."

DYNAMICS CHANGED

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Republican dominance has been so strong here that Democrats ran no candidates in the last two U.S. House elections, even though state voter registration records show Democrats outnumbering Republicans.

But dynamics have changed in the district, which runs from wealthy suburbs south of Pittsburgh through hardscrabble steel and coal mining towns to farmlands along the West Virginia border.

Saccone, 60, a conservative who has described himself as "Trump before Trump was Trump," led the race by more than 10 percentage points in January. The contest has since narrowed to a toss-up on a wave of Democratic voter enthusiasm for Lamb.

A Monmouth University poll on Monday showed Lamb ahead.

Bob Zelleznick, 59, of Bethel Park, said he intends to vote for Lamb and hopes for a Democratic win that would make the White House sit up and take notice.

"This is important because I think it will send a message to Washington: a lot of people are unhappy with the administration,” Zelleznick said.

The White House has arranged a string of visits to energize Saccone's base. Trump himself has visited twice. The area has also seen Vice President Mike Pence, Trump adviser Kellyanne Conway, Trump's daughter and adviser Ivanka Trump and, on the eve of Election Day, the president's son Donald Trump Jr.

"When President Trump's in your corner, how can you lose?" Saccone told supporters at a weekend rally with the president.

A college professor and former Air Force counterintelligence officer, Saccone won an endorsement from the influential Pittsburgh Post-Gazette newspaper, which praised his greater experience and knowledge of the district.

“He supports small business owners and would do anything for you to make sure that your business is successful,” said Stephanie Squibb, 48, of Elizabeth, who owns a local printing company.

But Saccone has not had much luck running on the Republican national agenda. Tax cuts, the Republican Party's only major achievement under Trump, have done little to energize local voters, some of whom dismiss the sweeping tax overhaul as a giveaway to the wealthy.

Republicans see the president's plans to impose steel and aluminum import tariffs as a way to generate enough enthusiasm among Trump's blue-collar supporters to counteract the energy of Lamb's backers.

"They're hoping this is something that potentially, in the grand calculus, could throw Saccone over the top," said Republican strategist Ford O'Connell.

While the district has just over 500,000 registered voters, U.S. Labor Department statistics show that metal production accounts for less than 10,000 jobs.

In a pair of Fox News television interviews on Tuesday morning, Saccone acknowledged voter turnout would be key and vowed to help Trump implement his agenda.

"He needs some help down there. He's getting beat up in Washington as you see from the media and from the bureaucracy and from Hollywood," he told Fox Business Network.

UNUSUAL DEMOCRAT

Lamb, who has the backing of unionized steelworkers, says he agrees with the planned tariffs.

His campaign claims to have reached more than 100,000 households with pledges to protect union jobs and pensions along with Medicare and Social Security programs for the elderly that account for nearly one out of every five local residents.

Republicans have also found it harder than expected to mount effective attacks on Lamb's positions on abortion, guns and the national Democratic Party.

He has eschewed the national Democratic brand, saying he would not support House Democratic leader Nancy Pelosi as speaker. But Lamb did get help from former Vice President Joe Biden, a favorite among Rust Belt Democrats.

Lamb says he personally opposes abortion but accepts the Supreme Court's 1973 Roe vs. Wade landmark decision allowing abortion as the law of the land. He favors enforcing existing gun laws and improving the current system of background checks over setting new gun restrictions.

The contest to replace Republican Tim Murphy, who resigned last year, is the fifth competitive special House election since Trump took office.

Democrats achieved a victory in Virginia and a Senate upset in conservative Alabama. Tuesday's election has little bearing on the current balance of power in the House, but Democrats hope a win would boost their momentum as they try to pick up the 24 seats they need to gain control in November.

Of the 85 races currently viewed as competitive, 55 are in congressional districts that Trump won in 2016, according to the University of Virginia Center for Politics.

Lamb has dramatically outraised Saccone by pulling in $3.9 million since last October.

Outside Republican groups, led by the National Republican Congressional Committee and Congressional Leadership, Fund have spent at least $8.1 million to keep Saccone afloat, according to the Washington-based Center for Responsive Politics, which tracks money in campaigns.

Democratic groups have spent about $1 million to oppose Saccone in support of Lamb, the Center said.

Pennsylvania's congressional district lines have been redrawn for November by the state supreme court, which will require Lamb and Saccone to run again in separate new districts if they hope to serve in Congress after 2018.

(Additional reporting by Susan Heavey in Washington; Editing by Alistair Bell and David Gregorio)

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