This time around, Republicans say they simply have better candidates in a much better environment. “I spent 28 years in business prior to getting involved in politics a couple of years ago, and learned some principles: Keep your skin thick and your heart tender,” Mr. Daines said with a shrug.

They say this year will be more like 2010, when Representatives Mark S. Kirk of Illinois, Roy Blunt of Missouri and John Boozman of Arkansas cruised to Senate victories.

“One election cycle does not a trend make,” said Brad Dayspring, a spokesman for the National Republican Senatorial Committee. “Actually, to the contrary, the fact that there are 51 former members of the House currently serving in the Senate suggests that 2012 was an outlier.”

To keep the Senate, Democrats must prove that wrong — and tar House Republicans seeking to jump to the upper chamber with the sins of a Congress with record low approval ratings.

“What you’re seeing here in Arkansas is, once again you’ve got a fundamentally flawed candidate who is trying to convince Arkansas voters that they should look past his record in the House as a reckless and irresponsible member who has sided with the special interests bankrolling his career,” said Erik Dorey, a spokesman and strategist for Senator Mark Pryor, Democrat of Arkansas, in an attack on Representative Tom Cotton, Mr. Pryor’s 2014 opponent. “He can’t run away from his votes.”

The approach is not going to work, said David Ray, communications director for the Cotton campaign.

“Those candidates don’t have any bearing on our race,” he said of the failed 2012 stable. “What we’re trying to do is distinguish ourselves from Senator Mark Pryor, and he makes that easier for us each and every day.”