Party strategists say they have taken steps to build a relationship with Mr. Sanders and his organization, and a top Sanders lieutenant, Jeff Weaver, attended a recent briefing hosted by the Democratic Congressional Campaign Committee, along with representatives from Planned Parenthood, the liberal group Swing Left and the centrist think tank Third Way, according to a person involved in planning the meeting.
But Mr. Sanders and his supporters have continued to seek out victory on their own terms — so far with little success — by venturing into party leadership races, primaries and long-shot special elections that establishment Democrats have avoided. The biggest test so far of Mr. Sanders’s clout may come on Tuesday in Virginia, where he has backed Tom Perriello, a liberal former congressman, in a contested primary for governor.
Still, even some Democrats competing in difficult elections have taken up ideas once associated with the hard left. Doug Applegate, a retired Marine colonel who narrowly lost a race last year to Representative Darrell Issa, Republican of California, said he would endorse single-payer health care in a new bid for Mr. Issa’s affluent coastal district.
“Single payer has become a moral issue,” Mr. Applegate said, adding he would be delighted to campaign with Mr. Sanders.
Others are warier: Representative Emanuel Cleaver, a Missouri Democrat and former chairman of the Congressional Black Caucus, said the party should give “some leeway” to candidates to match the politics of their districts. Mr. Cleaver said he recently ran into former Representative John Barrow of Georgia, one of the last moderate white Democrats elected from the South, and recalled telling him, “We’ll know that we’re on the winning track when you can get back to Congress.”
“We are going to lose every possible winnable seat, in a year where there are many winnable seats, if we come across as inflexible left-wingers,” Mr. Cleaver said. “I respect Bernie — I just don’t think we can become the party of Bernie.”