PITTSBURGH — The special election for a southwestern Pennsylvania House district tests three big questions hanging over the 2018 campaign. Can tax cuts help the Republican majority in Congress turn back the Democratic tide?

Can Democrats compete in Republican districts they need to recapture the House?

And can President Donald Trump rally his blue-collar base behind his party in November? As voters head to the polls on Tuesday, the answers appear more encouraging to Democrats.

The role of tax cuts

In a district Republicans have held for 15 years, where Trump won by 20 percentage points, Democrat Conor Lamb has eliminated Republican Rick Saccone's lead in the polls. Strategists for both parties call him the favorite to win. Here as elsewhere, Republicans have counted on their $1.5 trillion tax cut to counter voter discontent with the turbulence surrounding Trump. Not only does it show that all-GOP government works, they argue, it puts money in the pockets of middle-class families. Republicans have spent millions advertising that message in ther district, which includes parts of Greene, Washington, Allegheny and Westmoreland counties. What they've found is that however pleased families are about slightly higher paychecks, it is not moving voters in large numbers. That's partly because this race got underway after the previous congressman, Rep. Tim Murphy, resigned amid a sex scandal last October — before the tax cut passed. And it's partly because Lamb neutralized the issue while making this a local, not national choice.

Local vs. national

Lamb says he, too, favors tax cuts — but for the middle class, not the wealthy. He calls the Trump tax cut too expensive, warning elderly voters that the higher deficits it produces will lead Republicans to cut Social Security and Medicare. The trim 33-year-old Democrat benefits from the personal contrast with his stocky 60-year-old Republican rival. A former federal prosecutor and Marine, who shoots a rifle in one campaign ad, Lamb identifies himself with hometown values of service and sacrifice. Saccone stands so closely to Trump that he names only one disagreement: He roots for the Pittsburgh Steelers, while the president backs the New England Patriots. But Lamb, deflecting attempts to call him a lackey for Nancy Pelosi, says he won't support the liberal San Franciscan to remain House Democratic leader.

The Trump factor

Treading carefully in a Trump stronghold, Lamb has campaigned beside national Democrats with moderate reputations, like former Vice President Joe Biden, a Pennsylvania native who grew up in Delaware. He doesn't mention Trump's name. But Lamb still benefits from the backlash against the president's turbulent debut in office, especially among women in suburban areas. Even some working-class men who voted for Trump have decided that was a mistake. Given the size of the president's 2016 majority, Saccone can afford to lose some of Trump's base. At a campaign rally with Trump over the weekend, he tried to rouse enough of them to win. The president also offered Saccone a last-minute boost of administration policy. The tariffs on steel and aluminum Trump has announced are tailor-made for blue-collar neighborhoods outside Pittsburgh, though the once-fabled "Steel City" has diversified its economy around sectors such as technology and health care in the 21st century. But there's no sign the Republican president's move has reversed Lamb's momentum. For one thing, Lamb has himself offered qualified support for the tariffs — just as he has for some other controversial Trump initiatives such as a military parade in Washington on Veterans Day.

President Donald J. Trump with Rick Saccone speaks to supporters at the Atlantic Aviation Hanger on March 10, 2018 in Moon Township, Pennsylvania. Getty Images