Trump tries to stop the Democratic wave from crashing Pennsylvania

Trump visited Coraopolis, northwest of Pittsburgh. While the White House insisted his speech at H&K Equipment Company wasn't a political trip, Trump clearly had the race to replace the retired Republican Rep. Tim Murphy on his mind.

The GOP candidate in the 18th District race, Rick Saccone, greeted Trump at the airport. Later, on stage, Trump called Saccone "a spectacular man," gave him thumbs-up and thanked him for attending.

Saccone, a state legislator, faces Democratic Marine Corps veteran Conor Lamb in the March 13 special election.

Trump won district by 20 points

Several factors have elevated the race's importance: It's the first special election for a congressional seat of 2018, the first test of whether the GOP tax bill will persuade voters to stick with the party, and it comes at a time when senior Republicans are still determining whether to run for re-election.

In almost any other year, Republicans would be all but assured to hold the seat. It's a district that Trump won by 20 points in 2016.

But after a series of Republican losses -- the party's candidates were defeated in the Virginia and New Jersey governor's races in November, the Alabama Senate special election in December and a host of state legislative contests in between, including a state Senate race in Wisconsin on Tuesday -- GOP leaders and operatives are watching the race closely.

Republicans spending heavily

Conservative groups are already investing heavily in the race, with the Joe Ricketts-funded 45Committee pumping $500,000 into ads and Ending Spending saying it will spend $1 million.

The Congressional Leadership Fund, a Paul Ryan-aligned super PAC, is also spending on ads and organizing efforts. It is closely linking Lamb to House Democratic leader Nancy Pelosi -- even though Lamb has said he wouldn't support Pelosi for House speaker.

Democratic blog Daily Kos on Thursday announced its endorsement of Lamb, opening the door to what could be a flood of largely small-dollar donations from progressives.