Conservative Group Takes Sides in Michigan G.O.P. Primary

The country’s biggest conservative political organization announced Wednesday that it was diving into a bitter Republican primary in Michigan, its first such intervention into the broader battle between GOP-leaning business groups and Tea Party conservatives.

The group, Americans for Prosperity, will spend about $230,000 on advertisements thanking Representative Justin Amash, for fighting against President Obama’s signature health care law, officials there said â€" a shot across the bow of establishment donors who are rallying behind his challenger, Brian Ellis.

The ads do not attack Mr. Ellis, and officials at Americans for Prosperity said the commercial should not be construed as an endorsement of Mr. Amash. But the decision to get into a hotly contested primary on behalf of one of the most aggressive lawmakers hints at the delicate line the group is trying to walk: to maintain its credibility with other conservatives seeking to reshape the Republican Party without committing itself to an expensive internecine struggle that could endanger its chances of retaking the Senate this year.

“Congressman Amash has been a rock-solid vote for free-market issues in the House, and has been a leader on efforts to stop Obamacare,” said Tim Phillips, the president of Americans for Prosperity. “A.F.P. is well known for holding members accountable when they vote the wrong way; we think it’s also important to thank those who do the right thing and stand up against big-government laws like Obamacare that are hurting so many American families.”

Americans for Prosperity, co-founded by the billionaire David Koch, has spent about $27 million in recent months, most of it attacking Democratic candidates for the Senate over health care. In the process, the group has become the Republican leadership’s most critical ally in the battle to win control of the Senate. And unlike other conservative groups that have criticized the party establishment, like the Senate Conservatives Fund or the Club for Growth, Americans for Prosperity has so far steered clear of contentious Republican primaries in states like Kentucky and Texas.

But in backing Mr. Amash, the group is siding with an outspoken lawmaker who has repeatedly clashed with his own leadership in the House of Representatives. Mr. Amash, whose libertarian politics are often compared to those of former Representative Ron Paul of Texas, has pushed to strip financing for the National Security Agency’s data-mining efforts and restrict the use of drones. He has voted against the most austere budget proposals, arguing that they do not go far enough to restrain government, and last year he voted against a spending bill negotiated by Republican leaders that ended the two-week government shutdown.

In November, a number of prominent donors from Michigan’s business community announced that they would be supporting Mr. Ellis, citing Mr. Amash’s tactics on the shutdown among other issues. Mr. Amash has also drawn criticism from Karl Rove, the Republican strategist.

But Mr. Amash also has prominent allies. Chief among them are member of the DeVos family, Michigan’s most influential Republican donors, who have given generously to Mr. Amash’s congressional campaigns as well as to Americans for Prosperity.

Mr. Amash is also being backed by other right-leaning groups. FreedomWorks has endorsed Mr. Amash through its political action committee and pledged to match money spent by business-oriented groups on behalf of Mr. Ellis. Last month, the Club for Growth began running ads attacking Mr. Ellis as a “big-taxing, big-spending, corporate-welfare-loving politician.”