During their first debate in a closely watched Congressional election, Democrat Conor Lamb and Republican Keith Rothfus spent much of their time treading carefully on a number of hot-button issues, and appeared intent on separating themselves from hardliners in their own parties. But differences did emerge on some bread-and-butter economic concerns including Social Security.

The two men met at the studios of KDKA-TV Monday afternoon, recording a debate that will air at 7 p.m. tonight.

Rothfus sounded a somewhat warm, if vague, tone on immigration, an issue that President Donald Trump rode to the presidency. Rothfus said immigration issues need to be viewed from an economic perspective. "We need workers in this economy," he said. "We've got to grow this population."

After the debate, Rothfus explained his position on immigration by saying he backed the Securing America's Future Act, a measure proposed by Virginia Republican Bob Goodlatte and backed by NumbersUSA, a group that favors adopting higher barriers to immigration.

Lamb eschewed some proposals that are boosted by his party's populist wing, such as free college tuition at public universities. "I have been just as critical of those on my side" as he has of Republicans, he said. "We have to be square with people about how we'll pay for those ideas."

Rothfus and Lamb agreed that there would be little purpose in further investigating recently confirmed Supreme Court Justice Brett Kavanaugh, despite calls among some Democrats to pursue allegations he had committed sexual assualt decades ago. Lamb also played down the possibility that President Donald Trump could face impeachment if Democrats win the House. Lamb said he's seen no evidence that would support impeachment, though he added an investigation by special counsel Robert Mueller should be allowed to complete its work.

Rothfus has long been critical of an investigation, and at the debate said continuing it would contribute to "gridlock." Asked why he didn't provide more of a check on Trump by calling him out for inflammatory statements, Rothfus said he didn't respond to Trump's every utterance "because, frankly, that would be a full-time job."

Lamb has also refrained from publicizing disagreements with Trump, though he seemed to turn the question on his rival. "I have to keep things positive and tell the truth to lead by example," he said, before criticizing Rothfus' campaign for a series of ads attacking Lamb.

The sharpest differences involved health care as well as Social Security and Medicare, the topics that have dominated the race so far.

Rothfus espoused his votes to repeal the Affordable Care Act, also known as Obamacare. Efforts to repeal the measure have been a central cause for Republicans, though Democrats have seized on concerns that repeal would remove protections for people with pre-existing conditions. Rothfus denied that, saying that the last Republican effort to overturn the measure contained "express protections for people with preexisting conditions."

Experts have taken issue with such characterizations. While Republican legislation would bar the outright termination of insurance coverage for poeople with such conditions, it would lift caps on premiums in those cases, meaning insurers could raise rates to the point where many Americans could no longer afford coverage.

For his part, Lamb called the ACA's pre-existing condition guarantees "an important provision to protect people."

"I still don't understand his position," Lamb said of Rothfus. He warned that the GOP's 2017 tax cuts would create deficits that Republicans would try to pay for by cutting the popular retirement programs. "Republicans are coming for Social Security and Medicare," he said.

Rothfus denied that, and said that the key to protecting those programs, which face large deficits in future years, was economic growth. Republican tax cuts and other policies, he said, would create jobs and thus generate future tax revenues.

The race pits a staunch social conservative with a decidedly moderate Democrat in the 17th District, which includes Beaver County and swaths of suburban Allegheny.

Rothfus was first elected in 2012, after newly drawn Congressional district lines pitted Democrats Jason Altmire and Mark Critz against each other in the primary. Rothfus, of Sewickley, then beat Critz in the general election. In Congress, he has been a fiscal conservative and was for a time a member of the die-hard House Freedom Caucus, though he left the group in the winter of 2016-2017.

Lamb, of Mt. Lebanon, rose to national prominence earlier this year, when he beat Republican Rick Saccone in a March special election to replace Congressman Tim Murphy in the state's 18th Congressional District. The result was heralded as a clear sign of Democratic momentum going into the pivotal midterm elections. In his brief tenure, he has struck a middle course, occasionally siding with Republicans on votes like a resolution commending immigration officers and a largely symbolic effort to make household tax cuts permanent.

The two men are the only two incumbent Congressman in the nation facing each other this November. That's the result of a redistricting ordered by the state Supreme Court earlier this year. A new map combined portions of Lamb's 18th District with Rothfus' 12th.

This time, however, redistricting augurs much better for Democrats. Though it broke narrowly for President Donald Trump in the 2016 election, the newly drawn 17th District is seen as friendly to Lamb. The district adds Lamb's own South Hills Democrat base to Allegheny County's northern suburbs, while shedding many outlying areas including Cambria County where Republicans have performed better.

A mid-summer poll showed Lamb with a lead in the low double-digits, and outside Republican groups – which originally showed signs of investing heavily in the district – have pulled back their spending.

Rothfus is still very much in the fight, however, and he had a visit from Vice President Mike Pence in June.

A second debate between the candidates is slated to air on WTAE-TV and be simulcast on 90.5 WESA-FM on Tuesday, Oct. 16 at 7 p.m. The election is Nov. 6.