She gained statewide popularity winning three full terms to the House of Representatives, where she voted against President Obama’s health care law, for a constitutional amendment to ban same-sex marriage, and against cap and trade. That is the type of Democrat who is considered able to win elections in red states. So when Ms. Herseth Sandlin recently announced that she would not run, it not only upended Democratic plans but spurred deeper reflection over exactly what kind of candidate should represent the party — one who adheres to the party’s core principles or someone moderate, even conservative, enough to appeal to more voters? The debate in South Dakota could set a guidepost for Democrats nationwide weighing candidates’ electability versus their ideological purity. South Dakota is among three states where the balancing act in choosing a candidate who reflects the party’s values, yet can attract enough voters to win, could be particularly important in next year’s Senate elections. There, and in West Virginia and Montana, the Democratic incumbent is retiring, but in all three states, voters went resoundingly Republican in last year’s presidential contest, suggesting that keeping the seat in Democratic hands will be an uphill battle. And at the same time, the Democratic incumbents who are vying for re-election in three reliably Republican states — Arkansas, Alaska and Louisiana — expect difficult challenges as well. That makes a total of six seats — precisely the number Republicans need to retake control of the Senate. Democrats say the split in their party between ideological activists and moderates is not nearly as pronounced as it is in the Republican Party, where some are blaming right-wing purists for the party’s disappointing showing in the 2012 elections. But as Congress and state legislatures tackle core liberal issues like gun control, health care and gay rights, Democrats are starting to engage in their own soul-searching. Several liberal groups, for instance, have said they might find candidates to challenge the Democratic senators who voted against tightening gun background-check laws. “There’s a substantial population of the electorate coast to coast that wishes the Democrats would elect candidates who are stronger on certain issues,” said James R. Fleischmann, who has advised several red-state Democrats including Senator Max Baucus of Montana, who is retiring. “But there’s not a powerful organized strain of purists trying to correct what they perceive to be the incorrect position of the party.” Here in South Dakota, with Ms. Herseth Sandlin opting out of the Senate race, the only declared Democratic candidate so far is Rick Weiland, a small-business owner who has said he would fight corporate interests. Mr. Weiland also favors same-sex marriage and universal background checks for guns, and he is concerned about the weakening of Social Security and Medicare. Mr. Weiland has the support of his onetime boss, Tom Daschle, the former Senate majority leader (also a South Dakota Democrat), and of the party’s more liberal base. But his candidacy has upset some in the Democratic establishment. Many South Dakota Democrats are hoping for a centrist to compete in a state where the number of registered independents has increased nearly 20 percent over the past five years, while Democratic registration has dipped 7 percent. Senator Harry Reid of Nevada, the current majority leader, had said he would prefer that someone other than Mr. Weiland run, according to Mr. Daschle. A spokesman for Mr. Reid declined to comment. Mr. Daschle said he believed that Mr. Weiland would be able to earn the establishment’s support. “I’ve been through this hundreds of times — a candidate has to prove himself or herself before they get support of the D.S.C.C.,” he said, referring to the Democratic Senatorial Campaign Committee in Washington. “I believe Rick will be able to do that.” But Jason Frerichs, a state senator who leads the Democrats’ seven-member minority caucus, said, “We are a state that comes to the center.”
