(Mike Blake/Reuters)

So it looks as if Hillary Clinton is on her way to winning the Democratic presidential nomination. Despite speed bumps, her efforts to position herself as President Obama’s heir apparent seem to be paying off. That brings with it some challenges, which we can study in the presidencies of two previous “heirs”: George H.W. Bush, arguably elected to continue the Reagan Revolution, and Harry Truman, who became president when FDR died and was expected to continue and expand the New Deal’s liberalism.

Can the “heir apparent” win a general election?

The first challenge, of course, is getting elected. It’s not easy for a party to win again after holding the presidency for two consecutive terms. Neither Al Gore, Bill Clinton’s heir apparent, nor Hubert Humphrey, who was Lyndon B. Johnson’s, managed it. To win as Obama’s heir apparent, Clinton must reassemble the electoral coalition that put him in the White House.

Usually, an heir apparent finds that difficult. If elected, it’s by a smaller margin than the predecessor’s. We can see that in Clinton’s struggles to win over one of Obama’s key constituencies: younger voters. The possibility of Donald Trump as her Republican opponent may give Clinton the opportunity to reach out to some voters who did not support Obama, but her top priority must first be reassembling her predecessor’s coalition.

The potential heir usually takes that position by promising continuity. After a lethargic start to the 1988 campaign, George H.W. Bush found his voice as Reagan’s heir apparent by vowing to protect Reagan’s historic tax cuts from the Democratic lords of Capitol Hill. Bush didn’t emphasize new tax cuts; he promised to protect Reagan’s hard-won gains. That’s what’s expected from an heir apparent: defend the predecessor’s achievements, instead of launching new political revolutions. Clinton is promising that continuity, as opposed to Sen. Bernie Sanders’s promised “political revolution.”

How closely would Clinton hold to Obama’s policies?

If elected, an heir’s next challenge comes in how to relate to her predecessor’s approach. Clinton will probably promise a great deal of continuity in the general-election campaign. But if she reaches the White House, she — like preceding heirs — will probably alternate among three approaches: continuity, expansion and correction.

Continuity. Clinton will surely promise to preserve, protect and defend — with her veto pen if necessary — a variety of Obama administration initiatives from the Affordable Care Act to the Dodd-Frank financial regulation. Right now, according to the Charlie Cook Political Report and similar sources, it looks as if Republicans will maintain control of one or both chambers of Congress and most statehouses in the 2016 elections. As a result, a Democratic president would face a lot of resistance.

Truman practiced continuity when he vetoed the Taft-Hartley Act of 1947. This law was enacted by a Republican Congress and was intended to weaken organized labor. Taft-Hartley barred the “closed shop,” a workplace where only union members can be hired. And it enabled states to choose to pass “right to work” laws, which enables anyone to refuse to join a union, even if that union negotiated the operating contract. Truman vetoed it because he believed it would weaken the National Labor Relations Act, a key piece of Roosevelt’s New Deal agenda. Congress overrode Truman’s veto.

Expansion. But all presidents want to have their own achievements. Truman tried to both extend FDR’s New Deal and make his own mark through a legislative program he called the Fair Deal, which met with mixed success in Congress. Truman expanded on New Deal commitments by winning a higher minimum wage, increased Social Security benefits and a major new housing initiative. His proposals on national health insurance and repeal of Taft-Hartley went nowhere.

Truman also began fusing the New Deal’s commitments to economic justice with the cause of civil rights. His legislative proposals on civil rights stalled in Congress, but Truman was able to use executive orders to integrate the armed forces.

Clinton is likely to propose finishing Obama policy goals, like comprehensive immigration reform, legislative action on climate change, free community college and expanding the Affordable Care Act to help the millions not yet covered by this legislation.

Clinton has vowed to protect the Dodd-Frank Act, which regulates the financial industry, and to “fight for tough new rules, stronger enforcement and more accountability that go well beyond” that legislation. In addition to promising a rigorous enforcement of Dodd-Frank, Clinton may call for stricter standards upon executives and financial institutions. Republican opposition to all these is likely to be very strong, forcing Clinton to make many compromises and choose her battles accordingly. This is what she is hinting at when she calls herself “a progressive who likes to get things done.”

Again, we can learn from the past. Various aspects of Truman’s ambitious domestic agenda were defeated or trimmed by a coalition of Southern Democrats and Republicans. Bush was surrounded by an overwhelmingly Democratic Congress, but he could follow his predecessor’s footsteps by signing into law the Immigration Act of 1990, which largely built from the reforms of Reagan’s 1986 immigration legislation. Far more often, however, Bush was forced into the corrective stance.

Correction. Sometimes an heir has to take a new direction — whether by choice, political necessity, or problems in the predecessor’s policies.

Budget deficits grew steadily in the 1980s, reaching more than $220 billion by 1990. Faced with skyrocketing budget deficits and an overwhelmingly Democratic Congress, Bush had to move from continuity to correction on taxes. He abandoned his well-known campaign promise — “Read my lips. No new taxes” — and agreed to a package of tax increases. Fellow Republicans were angry.

Bush also had to clean up the mess left by Reagan’s deregulation of the savings and loan industry. Once rigorous regulations were moved, these financial institutions handled deposits recklessly and criminally, for years. Once they failed, Bush’s administration had to bail out depositors, costing taxpayers billions.

The elder Bush was challenged from the right in the 1992 Republican primary; while he easily beat Patrick Buchanan for the nomination, he did not win a second term.

If elected, Clinton, too, will most certainly have to correct something in Obama’s policy legacy, either because it’s malfunctioning or unpopular.

The Affordable Care Act, for example, contains a deeply unpopular “Cadillac tax” on expensive health-insurance policies, scheduled to go into effect in 2020. A future president Clinton would face enormous pressure to delay or repeal the tax. The problem is that the budget — and the ACA — depend on that revenue. How would she make it up?

What’s more, a Republican Congress is likely to keep challenging the ACA and may force her to weaken some of its important regulations, whether the individual insurance mandate or the contraception mandate. Or Clinton could refine and adjust Obama’s Trans-Pacific Partnership trade agreement.

Whichever one of these corrections she undertakes, Clinton — like her heir predecessors — will be in a thankless position, as was Bush 41. Her Republican critics would give her little credit for changing course. Her Democratic allies would probably claim that she lacks the spine for a fight or that she is “betraying” Obama’s progressivism, or — in the TPP case — would be unmollified that she’s holding to it at all.

What’s the takeaway?

Any heir apparent president after a comparatively popular or successful two-term president will probably face a stronger opposition party and a weaker and more divided governing party than did her or his predecessor. And no matter what he or she does — continuity, expansion or correction — that successor will be accused of lacking ideological coherence and core principles.

Bush was defeated in 1992. Truman left office very unpopular. Three other “successor” presidents — John Adams, Martin Van Buren and William Howard Taft — left office under similar circumstances. If she makes it to the Oval Office, Clinton can be a successful president. But she’ll face constraints that Obama did not.

Donald A. Zinman is an associate professor of political science at Grand Valley State University in Allendale, Mich. He is the author of “The Heir Apparent Presidency” (University Press of Kansas, 2016).