Immigration reform is the most important policy question facing Republican lawmakers as they begin their August break. But it's not the hottest topic in the GOP political world, not the one that dominates talk radio and the daily debate. That is the move to defund Obamacare.

With immigration, Republicans have the power to determine the outcome of reform legislation that has passed the Senate but still must make its way along a difficult path in the GOP-controlled House. Obamacare, on the other hand, is already law, and Republicans, without control of the Senate or White House, can't do much to stop the flow of health care subsidies set to begin January 1, 2014.

So the question is, when GOP lawmakers are home in August, will they devote more attention to the issue they can control, or the one they can't?

For the moment, it appears the answer is the one they can't.

It's an oddly topsy-turvy moment in GOP politics. Many Republicans appear to think immigration reform is nearly dead, even though it could well become law if things go as its advocates hope. At the same time, they see Obamacare as something they can still change, even though it is heading inexorably into implementation in the next few months.

"There's definitely more interest right now in Obamacare than immigration, partly because folks believe immigration has been stopped or slowed," says conservative radio host Bill Bennett of the calls he receives from listeners. "The passion against Obamacare never subsides."

There are good reasons for continuing Republican anger about Obamacare. The law was passed over unanimous GOP opposition during a brief window in 2009 in which Democrats had a filibuster-proof majority in the Senate. If even a few Republicans had supported the legislation, the fight today would be different.

But Democrats did it entirely on their own, and some are realizing that the way they passed Obamacare contributes to continuing opposition. "With the battle over health care still raging," the New York Times' Robert Pear reported last week, "some lawmakers say they now understand what Thomas Jefferson meant when he said, in 1808, that 'great innovations should not be forced on slender majorities,' or enacted without broad support."

In addition to that perhaps permanent Republican resentment, each day also brings news of some serious problem with Obamacare, lending credence to Democrats who have said they fear its implementation will be a "train wreck." And adding even more to Republican anger in the states is the hypocrisy of lawmakers who passed Obamacare but don't want to be subjected to its provisions.

Until last week, for example, it appeared that members of Congress and their staff, who today have generous coverage through the Federal Employees Health Benefits Program, would be subject to the same treatment as millions of Americans who will seek coverage on the Obamacare exchanges. That was a future Democrats simply could not accept; when they met with President Obama last week, the first thing they did was to ask him to exempt them from their own law. He did, and now members and staff will keep their generous coverage. Everybody else will have to hope for the best.

It's entirely reasonable for Republicans to be angry about things like that. But GOP activists should also keep in mind what they can change and what they can't. And at the moment, the thing they can change is not Obamacare but immigration reform.

If August goes quietly on the immigration front, some Republican lawmakers may return to Washington with the sense that voters back home don't really mind that immigration reform goes forward. And then it will. If, on the other hand, lawmakers hear expressions of serious opposition at town meetings, their conclusion will be just the opposite. And reform will likely go down to defeat.

So Democrats don't really mind if Republicans use up all their grass-roots energy railing about Obamacare. It's already the law. What would be a problem for Democrats, and for some pro-reform Republicans, is if the GOP grassroots concentrated its fire on immigration reform. That could well mean the end of President Obama's top legislative priority for his second term.

Immigration reform won't be taken up by the House until this fall. But its future will be determined in August, in town halls around America, where Republicans will either voice real opposition to the current proposals, or signal that they will go along. The next month will tell the story.

Byron York, The Washington Examiner's chief political correspondent, can be contacted at byork@washingtonexaminer.com. His column appears on Tuesday and Friday on washingtonexaminer.com.