In what has been one of the most competitive Massachusetts primaries of the 2012 election season, Republicans Sean Bielat, Elizabeth Childs, and David L. Steinhof will face off Thursday, Sept. 6 in the GOP primary in hopes of taking on the well-backed political newcomer Joseph P. Kennedy III in the newly redrawn Fourth Congressional District.

Bielat and Childs, the two frontrunners for the party nomination, have spent their campaigns highlighting their differences from Kennedy, who is expected to handily win his own Democratic primary on Thursday. Bielat, Childs, and Kennedy all hold Harvard degrees.

The Fourth Congressional District, represented for decades by retiring Congressman Barney Frank ‘61-’62, grew slightly in redistricting following the 2010 census. The district picked up several right-leaning cities in the southeast of the state which are expected to bring the new Fourth slightly to the right.

Bielat has focused his campaign on the economy, pledging to bring job-creating fiscal expertise and an academic eye to Washington, where he feels legislators are side-stepping the real problems facing the country.

“The lack of understanding of basic economics are a part of the problem,” Bielat said in an interview this spring. “So what you get are these really ideological arguments around policy implementation that really go on party lines.”

Childs, a Brookline physician, has presented herself as a more centrist Republican. Pro-choice and vocal about education reform, Childs has said she hopes to bring a physician’s eye to Congress to better evaluate health care reform and fiscal issues.

“Medicine is always weighing probabilities, uncertainties, risk and benefits and making decisions that have a major impact with limited information. If you think about it, that is what we need people to be able to do in government,” Childs said.

Lawrence S. DiCara ‘71, a former Boston City Councillor and longtime fixture of the Mass. political scene, said voter turnout in the Republican primary will likely be low because of a scarcity of registered Republicans in the district.

Todd Domke, a Boston-based Republican political consultant, saidthat it is likely that the southern, more conservative part of the district will back Bielat and the more liberal north will support Childs. Steinhof should earn some support from right-wing conservatives but is unlikely to win the nomination, Domke said.

Bielat has been considered the Republican favorite since he entered the race last fall. A former Marine and technology executive, he tried unsuccessfully to unseat Frank two years ago, drawing much national attention and financial support before ultimately losing by 11 points.

While Bielat was seen as a young rising star of the Republican party in 2010, his campaign has been muddled this time around by questions surrounding less-that-transparent campaign finance and personal history. But it was the emergence of another rising star, Kennedy, that really hurt the Republican frontrunner, experts said.

A former commissioner of the Massachusetts Department of Mental Health, Childs and her campaign suffered early in the race from a lack of visibility. In an unorthodox response, she spent the summer exploring the district in an Airstream Trailer, trying to maximize her exposure.

By far the most conservative candidate seeking the nomination, Steinhof’s campaign has struggled to gain traction.

Though the Fourth Congressional District is thought to have become slightly more conservative in the wake of redistricting earlier this year, DiCara said Kennedy will be the favorite over any Republican candidate nominated Thursday.

“I don’t think either of them comes close against Joe,” DiCara said.

—Staff writer Nicholas P. Fandos can be reached at nicholasfandos@college.harvard.edu.