WASHINGTON — The House Ethics Committee said Thursday that it would conduct a preliminary investigation into the campaign finances of Representative Cathy McMorris Rodgers of Washington, the House’s highest-ranking Republican woman, who just last week delivered her party’s response to the State of the Union address.

Ms. McMorris Rodgers is the chairwoman of the House Republican Conference, the fourth-highest Republican post in the House.

The committee’s action was prompted by a recommendation for an inquiry from the nonpartisan Office of Congressional Ethics. Elliot S. Berke, Ms. McMorris Rodgers’s personal lawyer, pledged to cooperate.

“The congresswoman and her office cooperated fully with the O.C.E. during its inquiry and have already begun assisting the committee with its review,” Mr. Berke said in a statement. “We are confident that the committee will ultimately find that the allegations were baseless and that her office always followed all laws, rules and standards of conduct.”