DES MOINES — The race between Hillary Clinton and Senator Bernie Sanders , which voters will begin deciding a week from Monday, is not just about the White House anymore. It has intensified into an epochal battle over their vastly different visions for the Democratic Party .

Mr. Sanders, a New Deal-style liberal from Vermont, last week became the party‘s first top-tier candidate since the 1980s to propose broad-based tax increases. He argues that only muscular government action — Wall Street regulations, public works jobs, Medicare forall — will topple America’s «rigged» economy.

«Something is grotesquely wrong in America,» he said Thursday in New Hampshire, urging voters to deliver a landslide in November that would cow Congress into enacting his agenda.

Mrs. Clinton, a mainstream Democrat, has started contrasting herself with Mr. Sanders by championing a «sensible, achievable agenda» and promising to build on President Obama’s legacy in health care, the economy and national security. She is the classic continuity candidate: seeking support from blacks, Hispanics, women, union members and suburban voters, and proposing policies that are friendly to families and businesses — strategies that have defined the party since President Bill Clinton‘s election in 1992.

«The middle class needs income, not tax increases,» she said Thursday in Iowa.

The question of whether Mr. Sanders can turn the liberal wing of the party into a true force again in presidential politics will have its first test in the Iowa caucuses next Monday. He has driven Mrs. Clinton into a tight race here, according to polls, and he has a lead among likely voters in the Feb. 9 New Hampshire primary.

While Mrs. Clinton has many institutional advantages, in those states and the rest, Mr. Sanders has electrified huge numbers of young people — the party‘s future — with his critique of Democratic support for free-market capitalism and interventionist foreign policy. He appears determined to steer the party to the left, even though he is not a registered Democrat but rather an independent and a self-described democratic socialist.

But many Democrats are torn about whether his liberalism, or Mrs. Clinton‘s pragmatism, will be enough to win a general election.

«The early enthusiasm for Sanders reminds me of the McGovern and Mondale races, where two good men were only able to win one state each in their presidential campaigns,» said John Breaux, a former Democratic senator from Louisiana, referring to liberalism’s wipeouts behind George McGovern in 1972 and Walter F. Mondale in 1984. «Democratic voters don’t want that to happen again.»

But Democrats have also harbored reservations about the party‘s incremental approach to government since the 1990s. Mr. Clintonmay have led the party away from liberalism, but many voters never lost a taste for it, said Bob Kerrey, a former Democratic senator and governor of Nebraska.

«Prosperity made it easier to do,» Mr. Kerrey said about the party‘s acceptance of Mr. Clinton‘s economic and deregulatory policies. However, he added: «The free-market approach to solving problems was discredited by the Great Recession. Liberalism is now seen as a remedy to excessive reliance on the market.»

The two futures for the Democratic Party are not only about ideology. The candidates have political strategy in mind, too, against the 2016 Republican nominee.

Mr. Sanders‘s ideas and intensity could energize the party‘s base and, he believes, inspire a tidal wave of support from young people, ensuring a level of voter turnout that would favor the Democratic nominee. (Low turnout in 2014 helped many Republican candidates in House and Senate races, he notes regularly.)

Yet Mr. Sanders risks easy caricature from Republicans as a tax-and-spend liberal who would turn the United States into a Scandinavian-style welfare state. And that could also hurt him with moderate Democrats.

«Sanders‘s ideas are deeply felt, but at the same time he has really overreached,» said Drew Westen, a professor of psychology at Emory University and a message consultant for congressional Democrats. «The average American is not going to buy into a vision of the federal government running one big health care program. Many people are actually afraid of that idea.»

Mrs. Clinton, by embracing many of the policies of Mr. Obama and her husband, is aiming to rebuild the same coalitions that elected them, and hoping that the prospect of the first female president will draw even more women to the Democratic side this time. While Mr.Sanders would have to defend his far-left plans, Mrs. Clinton believes the Republican nominee, whoever that may be, will face the challenge of defending radically conservative ideas.

But Mrs. Clinton‘s vision for the Democratic Party also comes with vulnerabilities: Many young people and liberals, and some women, believe her views are too moderate, and they do not trust her to fight for typical Americans. And Republicans will easily rally their voters if Mrs. Clinton, whom they have long demonized, becomes the Democratic standard-bearer.

«There are so many Democrats who just don’t believe the Clintons are true believers in the grass-roots progressive movement, and her ideas for the party aren’t really changing that,» said Douglas Brinkley, a presidential historian at Rice University. «She’ll need huge passion and turnout among Democrats to fight the Republicans’ desire to finally beat her.»

It is Mr. Sanders‘s message, not Mrs. Clinton‘s, that has had a more rousing effect on the voters who have heard those candidates the most over the last nine months: Democrats in Iowa and New Hampshire.

Part of Mr. Sanders‘s magnetism is that he has been raging against income inequality and Wall Street capitalism for years, and Vice President Joseph R. Biden Jr. recently observed that Mrs. Clinton was «relatively new» to talking so much about these issues. Even some liberal supporters of Mrs. Clinton say the stagnant wages and other financial burdens weighing down many Americans have created a prime opening for Mr. Sanders to restart a debate over the Democratic Party‘s priorities.

«When you have a tax system that is so horribly skewed and loaded with loopholes as ours has become, and big companies don’t have significant taxes to pay, you’re seeing a lot of Democrats asking what their party is going to do about it,» said Michael S. Dukakis, the former Massachusetts governor, who was the Democratic presidential nominee in 1988.

«The vast majority of people should be doing better in this economic recovery,» said Mr. Dukakis, who is backing Mrs. Clinton. «‘Why aren’t we?’ they ask. Bernie is talking right to them.»

Yet both Mr. Sanders and Mrs. Clinton have serious political vulnerabilities that have prevented either of them from convincingly selling a vision to clear majorities of voters in Iowa and New Hampshire, despite months of trying.

Many Democrats question Mr. Sanders‘s ability to win the election. Though he does well in poll matchups against possible Republican nominees, the effects of his tax plans have barely started to sink in with voters. He also has little experience on foreign policy and has indicated he would resist intervening in overseas conflicts, raising concerns among some Democrats that he would focus on the economy at the expense of a strong American role in the world.

For his supporters, though, his vision for the party is inspiring. Erin Bilbray, a Nevada Democrat who ran for Congress in 2014, said she was willing to pay more in taxes to guarantee health insurance for her two children as they grew older. And she said Mr. Sanderswas better suited to upending an economic system that had sent many of her neighbors in Las Vegas into foreclosure, harming their community and driving down the value of her home.

«Being a Bernie Sanders Democrat means loving my country so much that I am willing to sacrifice to pay for its education and health care,» Ms. Bilbray said. «The middle class is tired of being the doormat of corporate America.»

Mrs. Clinton struggles with longstanding negative feelings among some voters who see her as too close to Wall Street and too hawkish on foreign policy, such as her vote to authorize the Iraq war (which Mr. Sanders opposed). Put more simply, some voters do not like her or do not consider her honest. In a CNN/WMUR poll last week of likely Democratic primary voters in New Hampshire, 55 percent called Mrs. Clinton the least honest candidate in the party‘s field, compared with 46 percent in December.

But Kay Kubik, a retired farmer who went to hear Mrs. Clinton speak on Thursday in Vinton, Iowa, said she thought Mrs. Clinton was «sincere and experienced about supporting policies that help out Democrats and all Americans.»

«She isn’t in it only for herself, and she doesn’t get angry about all the nasty things that people say about her,» Ms. Kubik said. «She stays focused on the issues and what’s doable, and that’s what the party needs in a president.»

Hillary Clinton and Bernie Sanders Battle for Party’s Future

 PATRICK HEALY

The New York Times  January 24, 2016