A model that aims to capture historical turnout in midterm election years found a 2 percentage point lead for Lamb. A third, high-turnout model showed a 7 percentage point edge for the Democrat. Saccone led in all three models in a Monmouth poll released last month.

A Monmouth University poll of the 18th District race in southwestern Pennsylvania showed the 33-year-old former prosecutor with an advantage under three different voting models. Lamb leads 60-year-old Republican state Rep. Rick Saccone by a 51 percent to 45 percent margin if turnout matches the Democratic enthusiasm seen in special elections during the last year.

Democratic candidate Conor Lamb has seized the lead in Tuesday's closely watched House special election in Pennsylvania, according to a poll released Monday .

Conor Lamb, Democratic candidate for the U.S. House of Representatives, left, greets an attendee after speaking during a campaign rally with members of the United Mine Workers of America (UMWA) at the Greene County Fairgrounds in Waynesburg, Pennsylvania, U.S., on Sunday, March 11, 2018.

"This district has voted overwhelmingly Republican in recent elections, but a large number of these voters have blue-collar Democratic roots. Lamb seems to have connected with them," Patrick Murray, director of the Monmouth University Polling Institute, said in a statement.

On Tuesday, Republicans look to avoid a potentially embarrassing loss in a district Trump won by 20 percentage points in 2016. While the district will no longer exist as it is drawn now when Pennsylvania's new congressional map takes effect for November's midterm elections, the result of the race could affect fundraising, recruitment and party morale as the major parties battle for control of Congress this year.

The race has drawn national media attention, campaign stops from President Donald Trump and former Vice President Joe Biden and massive spending from Republican outside groups and Democratic donors. The election is considered a dead heat. Two separate polls of the race released this month showed each Saccone and Lamb with a three-point edge, within both polls' margins of error.

Here are some of the other findings in the Monmouth poll of the 18th District released Monday:

Among self-identified Democrats, 95 percent support Lamb and 5 percent back Saccone. Among those who say they are Republicans, 88 percent back Saccone and 9 percent support Lamb.

Lamb has a 51 percent to 45 percent edge among independents, the survey says.

Trump's move to put tariffs on steel and aluminum imports appears not to have done much to change likely voters' minds. Only 3 percent said the actions moved them toward backing Saccone, while 1 percent responded that the tariffs made them more likely to support Lamb. Ninety-six percent said the actions did not change their opinion on the race. Both candidates gave at least qualified support for the tariffs.

Likely voters were divided on Trump, with an even 49 percent saying they approved and disapproved of the president.

The telephone poll taken from March 8 to March 11 has a margin of error of plus or minus 5.1 percentage points.