For many conservatives, Mitt Romney wasn’t their first choice. Come November, he will be their only choice. The Republican Party is set to nominate Romney for president. Few will find Barack Obama an acceptable alternative, and even fewer will vote third party.

So what is a conservative to do? Right-thinking Americans would do well to consider the parable of the two Bushes. George Bush the elder was nominated to be Ronald Reagan’s successor, but conservatives never trusted him. They remembered his barbs about “vodoo economics” and his Eastern establishment roots. So when the first President Bush raised taxes, they rebeled. When he signed a quota bill, they rebuked him. They read his lips and then read him the riot act.

That presidency didn’t work out terribly well for George Bush. He served only one term. His share of the popular vote collapsed by 16 points in four short years. Desert Storm and expansions of government remain his most enduring achievements. But the resistance to Bush’s backsliding helped put conservatives in charge of the Republican congressional leadership. In 1994, the leadership won control of Congress and thwarted Bill Clinton’s most liberal initiatives. Conservatives gained in the long term by keeping the Republican president at a distance.

George Bush the younger knew how to talk like a conservative. He left behind his Yale roots and spoke with a Texas accent. He connected with evangelicals because he was one. And he had the good fortune to have a primary opponent named John McCain, who implausibly thought he could win the Republican nomination by running to Bush’s left.

Once in office, Bush cemented his bond with conservatives with his resolute response to the 9/11 terrorist attacks. So when he grew non-defense discretionary spending at twice the rate of Clinton, many conservatives looked the other way. The same held true when he doubled the size of the Department of Education — which even Bob Dole campaigned on abolishing — through No Child Left Behind. Bush’s Medicare Part D increased the program’s unfunded liabilities by trillions of dollars and was the largest new entitlement program since LBJ’s Great Society.

Republican failures endangered Republican successes. Out-of-control spending threatened to wipe out the Reagan tax cuts, with increases in the top marginal income tax rate coming in 1990 and 1993. Trillion-dollar deficits similarly endanger the Bush tax cuts, which will expire absent direct congressional action. But far fewer conservatives were complicit in the first set of failures than the second.

So which Bush will be the model for how conservatives treat Romney? My former American Spectator colleague Philip Klein hopes conservatives will stick to their principles, but he acknowledges it’s no sure bet. “It’s easy for Republicans to talk tough about fighting for smaller government when they’re in the opposition and a Democrat is in the White House,” Klein writes in his new book Conservative Survival in the Romney Era. “But the pressure placed on elected Republicans to sacrifice conservative principles becomes much more intense when one of their own is in power.”

Even rank-and-file conservatives will feel pressure to fall in line. Klein, now a senior writer for the Washington Examiner, notes:

In the coming months, those of us who criticize Romney from the right will be told we should save it until after November, or else we’re just helping Obama. When we do so after the election – should he win – we’ll be told he deserves a honeymoon period and needs to rack up a few accomplishments first before moving to items on the conservative agenda. Eventually, it will be that we can’t weaken him before the midterm elections, and then later, that we have to loudly support him, or else he’ll lose reelection to an even worse liberal boogeyman (or boogeywoman) in 2016.

But if candidate Romney needs only to woo swing voters, he will ignore conservatives. If President Romney faces pressure only from his left, he will surely disappoint the right. When Republicans take the base for granted, government gets bigger and conservative policy objectives go unrealized.

Despite his good intentions on Social Security reform, George W. Bush’s presidency was an enormous missed opportunity for conservatives on entitlements. Not many more opportunities will present themselves. Romney needs to have the same political incentives to govern in a conservative fashion that Tea Party challengers have created for complacent Republican incumbents across the country.

Klein was an early conservative critic of Romneycare, presciently realizing its individual mandate would be hard to confine to the state level — and that even many of the Massachusetts health care law’s conservative backers saw it as a model for the nation. He correctly observed that it would at the very least complicate the political case against Obamacare.

Yet in Conservative Survival in the Romney Era, Klein isn’t trying to dampen conservatives spirits ahead of November or get people to stay home rather than support the Republican nominee. He is writing an important blueprint for how conservatives can make a Romney presidency worthwhile.

“Even if conservatives would have preferred a different Republican nominee,” Klein writes, “there are still plenty of ways for them to advance their ideals by pressuring Romney into behaving more like the conservative for whom they had longed.” If they learn from Klein’s book — and the tale of the two Bushes.