GREENCASTLE — The final days of a five-way race between Republican candidates seeking the nomination to challenge Democratic U.S. Sen. Bob Casey is becoming increasingly bitter, with dueling attack ads running up the tab on a low-key election that is poised to exceed $5 million before Tuesday’s primary.

On Sunday night, Steve Welch and Tom Smith were to speak after GOP presidential front-runner Mitt Romney at a Franklin County Republican Party dinner as they scramble to connect with party faithful, many of whom had no idea who was running for U.S. Senate until the past couple of weeks.

“Most of the time, it’s Election Day before I make up my mind,” said Richard Beard, a dinner attendee from nearby Chambersburg who couldn’t name the Republican candidates.

Welch, a Chester County entrepreneur and venture capitalist who is backed by Gov. Tom Corbett and endorsed by the state GOP, is in danger of losing.

The campaign of Smith, a wealthy former coal company owner from Armstrong County, has released internal polling results that show him leading comfortably. And he is financing a barrage of TV ads that has him on track to spend more than three times as much as his four rivals combined.

Meanwhile, a third candidate, former state Rep. Sam Rohrer of Berks County, remains popular with conservatives who liked his efforts as a lawmaker to eliminate property taxes and supported him in an unsuccessful primary run against Corbett two years ago.

With weather forecasters expecting rain and snow today and Tuesday for much of the western half of Pennsylvania, turnout in an primary election already lacking excitement could be extremely low and make the result that much more unpredictable.

Casey’s perceived strength — he whipped incumbent Republican Rick Santorum in 2006 — also helped put the race on the GOP’s back burner.

Another dinner attendee, Duane Schroyer of Greencastle, said he planned to vote for Smith, but only after he asked to be reminded of who exactly was running.