Sen. Bernie Sanders (I-Vt.) speaks during a news conference following a weekly policy luncheon on Capitol Hill on Dec. 19. (Zach Gibson/Bloomberg)

Thanks mostly to President Trump, Democrats believe they are poised for good things in 2018: the possibility of taking control of the House and gains elsewhere in the midterm elections. But planning victory laps would be premature. Whatever their prospects for the fall campaigns, the Democrats are still in need of renovation and renewal.

Many current indicators point to rough days ahead for the Republicans, unless passage of the tax bill somehow changes their fortunes. From the president’s low approval ratings to the high energy among rank-and-file Democrats, as well as recent polls showing that the public prefers Democratic candidates for the House by a sizable margin, there is ample evidence that the GOP faces a typically bad midterm election year, or possibly worse. One caveat to all that: In the era of Trump, traditional metrics should not be taken for granted.

The Democrats need a net gain of 24 seats to take control of the House and a net of two to secure the majority in the Senate. The Cook Political Report lists 17 Republican-held seats as toss-ups and one as leaning to the Democrats. Another 22 GOP seats are in the “lean Republican” category, meaning they are at risk next year. In contrast, Cook’s team lists just four Democratic seats as toss-ups and five as “lean Democrat.”

The Senate remains a heavier lift, largely because Democrats are defending far more seats and have only a few opportunities to take away GOP-held seats.

The ample availability of competitive House districts is one reason there is a growing consensus, or at least a rising chorus among the political class, proclaiming a tsunami-in-the-making across America. If that turns out to be the case, Democrats would have the power to frustrate Trump’s and the GOP’s agenda while putting the president himself under a microscope. Many Democrats salivate at the prospect.

A Democratic takeover of the House would transform the politics of Washington. But would it necessarily represent a transformation of the Democratic Party? As with all midterm elections, particularly those that take place in a president’s first term, such a result would say much more about perceptions of Trump and his party than being an affirmation of the Democratic Party.

Despite the positive indicators about the midterms, Democrats face questions about their future as a party that now controls nothing in Washington and far less in the states than they did at the beginning of Barack Obama’s presidency. Among those questions are such basics as their agenda, their geographic limitations and their leadership.

Democrats could assume they can push those vulnerabilities to the sidelines during a midterm election year with a campaign message that is almost exclusively anti-Trump. But as even many Democrats acknowledge, something more than that will be needed to regain widespread trust of voters across the country and begin the process of rebuilding the party in places where it suffered losses over the past decade.

Democrats stand for many things that are popular with a majority of Americans. They oppose cutting tax rates for the wealthiest taxpayers. They oppose changes to Medicare and Social Security that would reduce future benefits or notably alter the eligibility requirements. And they want some immigrants, known as “dreamers,” to be able to stay in this country and not face the threat of deportation over the fact that they were brought to the United States illegally by their parents.

But there are hard questions for the Democrats. What exactly is their health-care policy likely to be in the future? Stand pat with the 2010 Affordable Care Act after some modifications? Move toward a single-payer plan, as Sen. Bernie Sanders (I-Vt.) and others now advocate?

What is their economic policy, other than rhetoric about helping working families? What is their response to concerns among many workers about the impact of globalization — more free trade or a rollback? What about cultural issues that are vitally important to a substantial portion of the party’s base but that play less well with others who have defected to the Republican Party?

Hillary Clinton learned in 2016 that a laundry list of programs does not necessarily translate into a compelling message. Democrats recoil at Trump’s “Make America Great Again” slogan as one that would take the country back to a time when women and minorities had fewer rights and opportunities, but Democrats also continue to struggle to enunciate a new America message that resonates powerfully, especially between the East and West coasts.

Senate Minority Leader Charles E. Schumer D-N.Y.), left, accompanied by Sen. Bernie Sanders I-Vt.), second from right, and Sen. Ron Wyden D-Ore.), right, speaks at a news conference Dec. 19 following a Senate policy luncheon on Capitol Hill. (Andrew Harnik/AP)

The party’s geographical challenges will be put to the test starting in 2018. One reason for Democrats’ optimism is that there are more than a dozen vulnerable House seats in blue states and several others in suburban areas in states Trump won but that have gone Democratic in the past. A fuller test of the party’s ability to rebuild will come in gubernatorial races in the Midwest.

Among states in that region with contests in 2018, Republicans hold the governorships in Ohio, Michigan, Wisconsin, Illinois and Iowa. The Democrats’ best opportunity will be in Illinois, their worst in Iowa. Democrats also must defend governorships in Pennsylvania and Minnesota. Those results, along with the outcome of legislative races in those industrial and Midwestern battlegrounds, will offer clues about the rebirth of the party.

The party’s leadership also is an issue of concern. In the House, the top three Democratic leaders are in their late 70s. In the Senate, the two top leaders are in their late 60s or early 70s. None shows signs of stepping back.

Among the party’s prospective presidential candidates, Sanders is 76, former vice president Joe Biden is 75, and Sen. Elizabeth Warren (D-Mass.) is 68. Who among them will run in 2020 isn’t known, but one issue for Democratic voters that year will be whether they are prepared to look to a different generation.

Howard Dean, the former governor of Vermont and former chair of the Democratic National Committee, has been a one-person chorus calling for a generational change in leadership for his party in 2020. He has said he would like to see his party nominate someone age 55 or younger, preferably 50 or younger. His argument is that the party needs a new-generation leader who can speak to the future more authentically than someone a decade or two older.

Dean noted that younger Americans — say, under 35 — are one of the most important constituencies for Democrats (those younger than 30 voted better than 2-to-1 for Virginia Gov.-elect Ralph Northam in November). But Dean recognizes that these younger voters are more loosely aligned to political parties than older generations, and organize and mobilize differently than past generations.

He believes it will be essential to find a presidential candidate who both reflects those attitudes and can energize those younger voters. “You’ve got to have a candidate who really turns people on, and I think somebody much closer to this generation would be this person,” Dean said in a telephone interview on Friday.

Democrats see a divided Republican Party led by Trump as an easy target for criticism. For now, that will remain the principal focus heading into the midterm elections. But as they begin what amounts to a three-year campaign cycle of midterm elections followed by a critically important 2020 presidential race, will Democrats be forthright in assessing and dealing with their own vulnerabilities?